Abstract: A device for maintaining constant a certain viscosity of an adhesive, that is used for pasting a spine of an inner book or a book cover, by adding a diluent has an adhesive container filled with an adhesive. A rotating conveyor roll is immersed in the adhesive. An applicator roll is positioned downstream of the conveyor roll. It receives the adhesive from the conveyor roll and applies the adhesive onto a spine of an inner book or a book cover passing by the applicator roll. A stirring apparatus is arranged on the container and has a stirrer immersed in the adhesive. The stirring apparatus stirs the adhesive received in the container and also measures the viscosity of the adhesive received in the container. A control unit with a computer is connected to the stirring apparatus. The control unit compares a measured value of the viscosity with a set-point value of the viscosity and meters the diluent to the adhesive in order to regulate the viscosity.