Abstract: A method of photographic printing including making a stepped exposure of a simulation of a properly exposed gray card for a film to be printed having particular D/log E characteristics, and the film-mask-colored element for film having a colored mask, for a number of different periods of time on at least one sheet of photographic paper having certain D/log E characteristics; processing each sheet of paper; comparing each such exposure step of a simulation of a properly exposed gray card to a gray reference to determine the proper exposure time from the step most closely approaching the gray reference; and measuring the intensity of the exposing light to obtain the proper intensity which, with the proper time, determines the exposure to be used to print all film having D/log E characteristics generally similar to the particular D/log E characteristics on paper having D/log E characteristics generally similar to the certain D/log E characteristics.

Abstract: An improved passive optical range simulator device used to test the ranging function of the optical radar ranger system by input to a coil of optical fiber having a mirror at its inner terminus that permits such coil to act as an optical delay medium. Improvement resides in inclusion of a partial mirror at the input end of the optical fiber coil to present multiple "target echo" reflections of the laser pulses back to the optical radar ranger system undergoing test.

Abstract: An optically focussed laser radar operating with a radiation wavelength of typically ten microns is focussed on an object to be observed. Doppler data is obtained with a continuous wave signal by mixing a reference of the transmitted signal with a signal reflected from airborne scattering centers. The system is particularly useful for clear air turbulence applications wherein aerosols such as dust and pollen serve as the scattering centers for reflecting the radiation. The radar converts to a pulse Doppler system when the depth of field exceeds the desired range resolution.

Abstract: The relative transverse velocity between a local station and a zone of reference or observation is measured by providing first and second directional radiation receiving devices at the local station, such devices being aimed along parallel paths extending to the zone of reference. Such paths are spaced apart by a predetermined transverse distance. The first and second radiation receiving means include respective first and second converting means for converting the received radiation into first and second electrical signals. Any transversely moving radiation feature produces corresponding signal features in such first and second signals, such signal features being staggered with respect to time. The lapse of time between such staggered signal features is determined by time lapse measuring means. The velocity of such transverse movement is inversely proportional to such time lapse.

Abstract: Light of two different wavelengths is passed through or reflected from a member of the body so as to be modulated by the pulsatile blood flow therein. The amplitudes of the alternating current components of the logarithms of the respective light modulations are compared by taking their molecular extinction coefficients into account so as to yield the degree of oxygen saturation. By adding a third wavelength of light, the percentage of other absorbers in the blood stream such as a dye or carboxyhemoglobin can be measured. Fixed absorbers reduce the amount of light that passes through or is reflected from the body member by a constant amount and so have no effect on the amplitudes of the alternating current components that are used in making the measurements.

Abstract: The present invention relates to an integrated circuit body suited to be applied for a TTL single reflex camera, by arranging at least two of the three circuit parts, namely the indication circuit part presenting an indication function, the light sensing circuit part presenting a light sensing function and the control-operation circuit part.

Abstract: A sighting lens has a refraction displacement of the image such that upon selected positioning of the lens on a weapon, such as an archery bow, and using a reference point therewith, a novel sighting procedure for hunting animals can be used which comprises aiming a selected distance below the image of the animal as it appears in the lens. Prisms are included to correct the line of sight which may not be true because of offset mounting of the sighting device. Increments of distance are provided on the lens for assisting the archer in aiming at a distance which is beyond normal shooting limits of that particular sight.

Abstract: A flame head for a flame photometric detector is disclosed comprising an explosion-proof housing having a combustion chamber therein defined by a fused quartz tube flame wall barrier. Means are provided for securing a fiber-optic bundle to the housing with one terminal end of the fiber-optic bundle positioned adjacent the fused quartz tube whereby light emitted from the combustion chamber can be transmitted via the fiber-optic bundle to a photomultiplier and associated electronic detection means remotedly located therefrom. A burner tip communicates with one end of the fused quartz tube and provides gas effluent and fuel through a passage therein coaxially aligned with the quartz tube while providing required air to four circumferentially spaced ports disposed about the passage. Means are also provided for purging fuel from the area immediately adjacent the terminal end of the fiber-optic bundle adjacent the quartz tube.

Abstract: A method for linearizing the measurement of the volume loading of a fluid sample when it is determined by measuring the forward scattered light. The scattered light measurement which measures the volume loading is divided by a value related to a measure of the undiffracted light after it has passed through the sample.

Abstract: A ring laser gyroscope having a triangular block configuration and means to dither the block about its line of symmetry. The triangular block is configured to be symmetrical about its axis of symmetry to reduce the temperature gradient across the instrument, which effect causes output signals in the absence of an input angular rate. An optical wedge attached to the output reflector of the triangular block and an external output prism form an interferometer by means of which the sensitivity of the device is reduced and the interferometer signal made equal and opposite to that of the laser cavity.

Abstract: Disclosed is an interferometric apparatus and method for the inspection and detection of overlay errors characterized in that two plane polarized laser beams are directed onto the surface to be inspected, the angle included by these two beams being dimensioned in such a way that the radiation generating the plus or minus first order of the diffraction pattern of one beam is parallel to the other beam, thus generating an interference field which in the absence of overlay errors consists of an homogeneous fringe pattern while in the presence of such errors the fringe pattern is locally distorted. In a first embodiment, a first component of a laser beam is deflected onto a viewing screen by a beam splitter while a second component passes through the beam splitter to a mirror. The position and angle of the mirror is determined by the first order diffraction characteristics of the grating (object).

Abstract: A method for determining the quantity ratio of two components of a multi-substance mixture whose absorption bands are adjacent to each other and overlap comprises using an interference filter which is rotated in a uniform rotation about a first axis through the filter plane which forms with a perpendicular erected on the filter plane a fixed angle .beta. which is not equal to zero. The filter is rotatably arranged about a second axis through the filter plane and the method is characterized by adjusting the angle .alpha. which the axis of rotation forms with the ray in the range of .beta. is less than .alpha. so that a signal corresponding to the first derivative of the spectral intensity distribution of the radiation transmitted by the substance mixture just passes through zero and a quantity correlated with this angle is used as a measure of the quantity ratio.

Abstract: A screw, particularly an extruder screw for rubber or plastics materials, comprising a plurality of hollow screw components juxtaposed in end-to-end relation, a coupling sleeve disposed over the or each joint between two adjacent screw components, each end of each screw component which is joined to another screw component having axially extending external splines on an external face thereof and the or each coupling sleeve having axially extending internal splines on an internal face thereof to engage with the external splines on the ends of the screw components to secure the screw components and the coupling sleeve or sleeves against relative rotation, and means, preferably a bolt coupling end members of the screw, to secure the screw components and coupling sleeve or sleeves against relative axial movement.

Abstract: A dough mixing and kneading device which comprises a horizontally disposed container including an elongate cylindrical space closed by walls at the ends of the container and three blades within the space extending lengthwise therein. The blades are secured to and driven by drive shafts protruding through the end walls of the container. The blades when driven rotate along circular paths radially spaced from each other, one of the blades rotating closely adjacent to the inside wall surface of the container thereby acting as a scraper. A lengthwise opening at the top of the container is provided for loading the container with dough and discharging the dough after mixing and kneading by rotating the container about its center axis. A fixedly mounted cover closes the opening when the opening is turned into its upwardly facing position.

Abstract: A device for cleaning articles such as jewelry by vibrating a container for a cleaning solution in which the articles are immersed at sonic frequencies with a motor which drives an eccentric weight. The cleaning device is made mainly of molded plastic parts which can be assembled with minimal need for tools so as to minimize manufacturing and assembly costs.

Abstract: A system for printing dot-matrix characters wherein a double column of dot forming elements is used, and no single dot forming element is required to print more often than once in any four column interval of printed text. Means for timing the actuation of the dot forming elements wherein the "on" time of the individual actuators can be longer than the time interval between columns is included.

Abstract: This invention relates to a wire matrix printer system using a plurality of unique print wire actuator mechanisms operable to print dots on a printing medium. The novelty is in each print wire actuator mechanism using an electro-mechanical combination including (1) a print wire actuator; (2) a permanent magnet collector; (3) a solenoid collector; and (4) a connector assembly holding the aforementioned together as a unit. The print wire actuator includes a print wire assembly having a wire member moved through a printing stroke under the unique combined force of (1) a torsion rod, and (2) a solenoid member. Additionally, the wire member is returned to the rest or starting position under the force of the torsion rod and a permanent magnet collector member.

Abstract: An electronically controlled token engraving apparatus, wherein a slide moves blank tokens from a blank token receiving station to an engraving station and, after engraving, to an ejection station, under the control of a digital logic system is disclosed. The slide includes a token receiving aperture that receives a blank token at the blank token receiving station. At the engraving station, a typehead comprising a plurality of radial type bars is manually rotated by an operator until the desired character type is located above the upper peripheral edge of the token. Thereafter, actuation of a switch causes a toggle press to press the end of the type bar and, thus, the type downwardly against the token causing a character to be engraved therein. Just prior to a character being engraved, the token is indexed a predetermined distance. After a character is engraved, the typehead is manually rotated until the next desired character type is located above the upper peripheral edge of the token.

Abstract: Apparatus for printing characters on at least one data carrier comprising a printing head movable along the data carrier, a line advance device for feeding the data carrier substantially perpendicular to the moving direction of the printing head, an ink ribbon feed mechanism for feeding an ink ribbon in relation to the data carrier, a cut-off mechanism for cutting off the data carrier, and a shaft rotatable less than 360.degree. in one direction by a reversible motor from a home position to an end position determined by the selected operation cycle and thereafter back to the home position by reversing the direction of rotation of the shaft. The reversal of the shaft is performed at different times during subsequent operation cycles to permit gears and cams on the shaft to move the printing head to positions determined by the number of characters to be printed on the data carrier, actuation of the line advance device, and actuation of the ink ribbon feed mechanism or cut-off mechanism.

Abstract: Apparatus for the timed transporting of forms over the writing beam of a printing device, comprising a transporting device mounted downstream of the writing beam for causing the forms to make a timed movement of advance. At least one brake flap is biased so that a free end of the brake flap engages, in use, a form passing over the writing beam, immediately upstream of the writing or printing area. A lifting device is associated with each brake flap for lifting the brake flap away from the form during transport of the form to thereby frictionally release the form.

Abstract: A removable finger grip adaptable to a wide variety and size of writing instruments. The finger grip is an elongate resilient body having a triangular cross-sectional shape and a cylindrical bore coaxial with the longitudinal axis. In order to accommodate a wide variety of writing instruments, the cylindrical bore has a plurality of ribs or ridges along the entire length of the bore for gripping instruments smaller in size than the diameter of the bore. The triangular shape provides three planar surfaces for gripping the device when installed on a writing instrument and also provides a thin web of material in the planar gripping surfaces between them and the cylindrical bore at the center of the planar gripping surfaces. This thin web allows the finger grip to expand for accommodating writing instruments larger than the cylindrical bore. The resilience of the finger grip allows easy removal from one writing instrument for use with another.

Abstract: A paint roller support tray including front and rear ends, a paint well adjacent to and below the rear end for storing a supply of paint, and an inclined surface tapering upwardly from the well to the front end for rolling excess paint off the paint roller. The tray includes a receptacle or socket for receiving a portion of the paint roller frame for thus supporting at least one portion of the paint roller above the well and with the paint roller handle resting on the front end of the tray thereby allowing excess paint from the roller to drip back into the well.

Abstract: A device for the roller application of liquid coating compositions such as paint is provided comprising;(a) a hollow body portion having an applicator opening at one end and a container opening at the opposite end; said portion containing;(1) a liquid feed tube extending from said applicator opening through said container opening;(2) a solid support member engaging the outside surface of said tube;(3) a rod reciprocatingly mounted in said solid support member and extending through the wall to the inside of said tube;(4) a flexible disc centrally mounted on said rod within said tube having a diameter substantially the same as the inside diameter of said tube;(b) a hollow handle portion contiguous with said body portion having pivotably mounted therein a trigger, said trigger connected by linking means with said rod for reciprocating said rod within said tube;(c) a container for said liquid coating composition mounted in said body portion at said container opening; said tube extending into said container; and(d

Abstract: A writing instrument in which a marking element is removably mounted in a casing and held in the casing by resilient means acting lengthwise of the writing instrument. A removable plug at one end of the casing holds the removable marking element under compression in the casing.

Abstract: For sealing metal annular elements on to a cylindrical ceramic member, e.g. a ceramic electrolyte tube in a sodium-sulphur cell without causing excessive hoop stress in the ceramic, the ceramic tube is formed with tapered inner and outer surfaces at one end, correspondingly tapered inner and outer annular elements are put over these surfaces with a soft metal, e.g. aluminium interface layer, the inner and outer annular elements having coefficients of thermal expansion respectively less than and greater than that of the ceramic, the assembly is then heated and the annular elements forced in the axial direction onto the ceramic whereby the ceramic material, on cooling, is tightly secured between the annular members.

Abstract: An apparatus for the releasable, force-locking connection of two telescopically arranged tubes, especially for clamping the steering housing tube in the steering fork tube of a bicycle or the like, comprising an expanding cone axially displaceable within the tube defining an inner tube by means of a threaded spindle. In order to avoid, during clamping, deformation of the inner tube and thus frictional connection between both tubes along only a circular engagement line at the lower end of the inner tube and to eliminate the effect of the unavoidable play between the outer diameter of the inner tube and the inner diameter of the outer tube, the outer surface of the expanding cone bears against wedges. The latter are each arranged in a respective guide slot formed at the inner tube, extending parallel to the lengthwise axis of the tubes and having a closed contour.

Abstract: A display device includes a vertical corner post (1 ) to which a pair of horizontally disposed header panels (2) are secured at their ends by means of a plurality of support tabs (28,29) struck from a pair of support strips (10,13) connected respectively through junction strips (6,7) to a pair of interconnected corner strips (3,5), the support strips being in substantially normal relation to each other, a plurality of support apertures (26,27) formed in a support panel (20) interconnected by inwardly offset junction panels (18) and double back end panels (16) with the adjacent end edge of the header panels so that insertion of the support tabs into the support apertures forms support means for the header panels, and a locking tab (31) formed on a support strip and inserted into a locking aperture (30) formed on the support panel to secure the panels in an interlocked relationhsip.

Abstract: An elongated structural member adapted to many uses is made in separable, coaxially aligned sections to permit the sections to be stored in a short space. The sections are readily secured together in end to end relationship to form a strong member which may take both tension and compression.

Abstract: An attachment device and assembly for effecting connection and disconnection between component parts in a simple but reliable manner. A first structure includes first and second substantially parallel spaced apart plates, a conically-shaped, conically threaded member having a given cone angle attaching the first structure to a second structure which operatively abuts the first structure. Each plate has an aperture formed in it, the apertures being larger than the cross-sectional area of the threaded member portions being received thereby, and the second structure having a surface portion operatively abutted by the threaded member so that forces are applied tending to push the surface portion and aperture edges apart. The angle formed between a line tangent to the aperture edges and a line in the plane of the surface portion is equal to the cone angle of the threaded member.

Abstract: An improved three-layer slab supported by a reduced-thickness elastic foundation for roadway applications is described. The intermediate layer of the slab is significantly thicker and more porous than each of the adjacent upper and lower layers, and serves as a thermo-insulating layer to prevent the propagation of extremely low temperatures to the elastic foundation of the slab. The slab has a thickness significantly greater than that of the foundation. Typically, the upper and the lower layers of the slab are formed from poured asphaltic concrete, while the intermediate layer is formed from compacted natural crushed stone or granulated broken stone sprayed with fluid bituminous or a bituminous-gasoline mixture.

Abstract: A method of construction whereby facilities can be provided for the docking of vessels or relatively large draft, all dredging operations being dispensed with.

Abstract: A flow control system for selectively restricting or blocking fluid flow in an open channel or culvert. The system includes a normally collapsed inflatable bladder secured in the channel. When collapsed, the bladder and its securing structure presents virtually no obstruction to fluid flow in the channel. The bladder is selectively inflatable by delivering an inflating medium such as a pressurized gas (air) or a liquid thereto, and, when inflated, provides a controlled obstruction to fluid flow in the channel. The system includes means controlling delivery of the inflating medium to the bladder and also provides for selective and positive withdrawal of the inflating medium for deflation of the bladder. The bladder is sized and shaped according to the contour of the channel for maximum efficiency in controlling fluid flow in the open channel.

Abstract: A dowel device for resin grouting, comprising a dowel with a wedge member located in an opening formed in the insertion end whereby an end portion is radially expanded and flared outwardly. The invention also includes a method of reinforcing a hard medium and a method of providing an anchorage in a hard medium.

Abstract: An earthen structure for holding back a body of water has a sloped surface which is paved with an overlapping arrangement of strips constructed of castable material. Each strip is formed by conveying castable material from a mobile receiving hopper that is propelled adjacent to the base of the earthen structure. A bridge conveyor projects laterally from the receiving hopper over the slope. The bridge conveyor deposits the castable material onto the slope forwardly of a spreader device that is advanced longitudinally relative to the slope by a prime mover. A scraper blade of the spreader device is positioned at a preselected elevation by a forwardly projecting sidewall which is connected to the blade and slidable on a previously formed strip. The sidewall maintains the castable material in front of the blade, as the advancing blade spreads the material in a longitudinal strip of a preselected thickness and having a lateral edge extending a preselected lateral dimension outwardly from the sloped surface.

Abstract: The base member of a mine roof prop is provided with a closed lower chamber and an adjoining open-top upper chamber in which the lower end of a post is slidably mounted, with means at the upper end of the post for engaging a mine roof. A body of liquid is supported in the lower chamber by flexible means, below which there is an air inlet. The base member also has a liquid passage connecting the upper part of the lower chamber with the lower part of the upper chamber so that when compressed air is delivered to the air inlet, the flexible means will force liquid up through the passage and into the upper chamber to raise the post, whereupon a valve can be closed to trap the liquid in the upper chamber.

Abstract: A tool driving attachment is provided for use with a tool driving machine having a rotatable powered driving member. The attachment includes a first tool driving member which is connectable with the powered member for rotation therewith. A second tool driving member may be guided and resiliently locked into and out of rotating engagement with the first tool driving member when the first member is rotating.

Abstract: An improved counterbore end cutting tool characterized by a unique construction wherein the shank driver, the cutting head, and the pilot shank are removably mounted together. Additionally, the cutter head is manufactured in a manner to provide a novel reversible feature which permits it to be reversed to cut from either end. The cutting head and the holder include a novel driving connection feature wherein the teeth at one end of the cutting head which are not being used, mate with appropriate recesses in the holder to provide the driving connection between the holder and cutting head.

Abstract: An improved and modified cheese stacking machine for forming stacks of individually wrapped slices of cheese. Individual cheese slices are wrapped, cut and delivered in the machine to a discharge assembly where the individually wrapped slices are stacked one atop the other. A cam shaft rotates as the machine cycles and mounts a plurality of cams which operate cam switches to cause discharge of the stacks. An electrically operated clutch is provided between the cam shaft and the driving means, and an electro-optical sensor is mounted for sensing each slice as it is delivered to the discharge assembly. A counter is connected to the sensor for accumulating a count indicating the slices stacked since the last discharge, and the clutch operated upon a predetermined count to cause the stack to be discharged.

Abstract: A parking system for cars has a plurality of parking shelves disposed horizontally and vertically on both sides of a central zone of vertical movement, one car being carried to and from the shelf by a carrier activated by its driving gear means to raise on vertical racks in the central zone and to perform its lateral movement in the parking shelf at its parking level where a series of rams make the racks discontinuous and from the shelves, on both sides, extensible rails extend their mobile parts beneath the carrier when it is arrived at its parking level; the driving means of the carrier being: the car itself, a motor of the carrier, or the car for the loaded carrier and the carrier's motor for the unloaded carrier; the unloaded carrier being stored either in a central basement or in parking shelves.

Abstract: An improved vehicle is described which is adapted both for the handling of refuse containers and for general utility purposes. The vehicle has a chassis, an elongated lifting frame supported by the chassis, and means for elevating a rearward segment of the frame with respect to a rear segment of the chassis. Refuse container engaging arms are supported near a rear segment of the lifting frame and a utility vehicle body is mounted on the lifting frame. The vertical position of the container engaging arms is varied in accordance with movement of the lifting frame.

Abstract: A water collecting reservoir, desirably located at a considerable elevation, is constructed and arranged periodically to deliver water downward to an annular generator chamber which includes a fixed outer circular shell and a complementary, inner rotary shell, the latter shell being fixed on a rotary horizontal shaft which may be constantly driven by the water through pistons affixed to the inner shell. The rotary shaft is arranged periodically to open and close a valve for timing the admission of measured quantities of water to drive the generator.

Abstract: Advance diffuser for an axial blower having an impeller of large diameter received in a substantially circular housing and an axis extending in a given direction, includes a suction box formed with an intake opening through which a medium is drawable by suction in a direction substantially perpendicular to the given direction of the blower axis, a plurality of swirl vanes mounted on respective, mutually parallel shafts in the suction box, the suction box having a pair of mutually opposing parallel walls wherein the shafts are rotatably mounted, a pair of casing surface sections mutually connecting the parallel walls and tangentially merging with and surrounding the substantially circular outline of the impeller housing, the suction box having another wall facing toward the impeller and formed with a suction opening, the parallel shafts whereon the swirl vanes are respectively mounted being disposed along an arc surrounding part of the suction opening, the arc having end points mutually connectible by a secant

Abstract: Each blade on an impeller disc comprises an impeller portion extending substantially radially of the disc and a centrally located inducer portion on the front side of the impeller portion, the inducer portion being angled from the impeller portion in the direction of impeller rotation. The impeller portion itself is mostly inclined in the direction of impeller rotation, with the angle between the impeller portion and the front face of the disc being about 90 degrees at its inner end and gradually decreasing toward its outer end, where the angle is in the range of from about 50 to 70 degrees. Preferably, these blades are arranged alternately with auxiliary blades which are essentially identical with the impeller portions of the first recited blades.

Abstract: A method of and apparatus for self-sustaining high vacuum in a high voltage environment. The method and apparatus employ ion pumping; the apparatus can be an integral part of high voltage equipment and acts to maintain a high vacuum condition in such equipment.

Abstract: A method and apparatus for transferring heat regeneratively in a rotating heat exchanger. The heat exchanger provides for the cold gas stream to be heated alternate compression and expansion with heat addition usually during the expansion phase of the compression-expansion step from a warmer fluid. The fluid to be heated is usually a gas, while the fluid releasing heat may be either a gas or liquid. The heat element is a folded member usually in the form of bellows mounted concentric with rotor shaft, with baffles within to provide for outward and inward movement of the gas. Due to the heat addition especially during the expansion steps, the gas to be heated gains in pressure and usually this pressure gain is work free, and in magnitude is similar to the constant volume heat addition pressure gain which is well known.

Abstract: This invention is a multiple diverse energy source driven energy integration and multiple use-point system: which includes system air pressure compensated variable pressure and volume delivery of compressed air from multiple air compression stations which discharge and store compressed air into an included interconnecting collection storage and distribution conduit multiple module grid system; of largest needed and commercially available size pipe to keep the pressure drop to a minimum, and from which the compressed air is withdrawn at multiple points of need; when and as needed, through synchronized dual-precision-controls to turn, at optimum RPM speed regardless of varying work loads, air motor drives for operation of conventional electrical generating equipment with varying customer-use-demand output work loads.

Abstract: A fuel injection pumping apparatus comprises a displacement piston which is located within the cylinder, an outlet from one end of the cylinder and a fluid pressure operable member mounted within a further cylinder for actuating the piston and causing fuel to be delivered through the outlet. Means is operable upon movement of the displacement piston to a predetermined position to terminate delivery of fuel through the outlet and valve means is provided to control the application of fluid under pressure to the fluid pressure operable member.

Abstract: A pump and valve arrangement includes a three-way valve at the juncture of an inlet line, an outlet line and a pumping chamber. The valve comprises a generally spherically shaped flow control member rotatable in a valve body and having an arcuate slotted passage in one side thereof adapted to communicate simultaneously with either the inlet line and pumping chamber or the outlet line and pumping chamber for alternate flow of material from the inlet line into the pumping chamber and then from the pumping chamber through the outlet line. Adjustable seat sleeves extend into the body into sealing engagement with the flow control member and a journal of the flow control member extends outwardly of the valve body and has a quick release crank attachment thereon. The pump and valve are operated in synchronism with one another by respective motors controlled from position responsive switches.

Abstract: A power-converting device has a generally-elliptical rotor which is mounted within a generally-cylindrical chamber. That rotor reciprocates as it rotates within that chamber; and interacting surfaces which are wholly separate from, displaced bodily away from, and in addition to the shaft of that power-converting device halt continued movement of that rotor whenever it reaches an end of its path of reciprocation, and then smoothly start that rotor moving back toward the opposite end of that path of reciprocation.

Abstract: The fan housing has a generally circular outer peripheral contour and is provided with a plurality of substantially identical equally spaced radial projections and has at each axial end of the housing a boundary surface and including a tubular part extending in the axial direction of the fan housing intermediate the two aforementioned boundary surfaces. A drive motor is centrally mounted by radial spokes at one axial end of the fan. A fan wheel driven by the motor at least partly surrounds the motor and is provided with fan blades accommodated in an annular flow channel defined intermediate the tubular part of the fan housing and the drive motor. The fan blades extend substantially all the way to the other axial end of the tubular part of the fan housing. The tubular part at its middle has a circular interior cross-section closely surrounding the outer peripheral boundary surface of the fan wheel.