Abstract: A non-electric double delay borehole downline unit is provided for delaying transmission of an explosive signal down a borehole and also delaying initiation of the charge within the borehole after the signal has been received therein. The double delay unit comprises a length of flexible impulse propagating tubing having first and second ends and an impulse propagating hollow channel extending along the length thereof, a non-electric high strength blasting cap containing a first delay element connected to the first end for initiating a borehole explosive charge, an impulse amplifying and transmitting cap containing a second delay element connected to the second end and a protective block having a plurality of elongated apertures with one of said apertures fully and snugly receiving the amplifying cap therein.