The Archimedian Screw Pump, 91 



should have as many threads as the conditions of ordinary 

 workmanship and convenience will allow. That for screws of 

 any size, say 6 feet or 7 feet external diameter, the width of 

 the chambers should not be less than i8 inches on the square, 

 and the diameter of the core one-third nearly of the external 

 diameter. These conditions allow four threads for a screw, 

 whose spiral angle varies between 20° and 30° ; three threads 

 for those between 40"^ and 50° ; and two threads for 60°. The 

 threads of the screws of the pumps upon which he experi- 

 mented, and which he considered was the proper form to use, 

 were made developable, by which term he meant a curved sur- 

 face that could be unwrapped, laid fiat, and inside a plane. 

 The surface of the spiral thread, as ordinarily used, lies at 

 right angles to the surface of the core, and if laid out 

 flat the external edges would have more surface than the 

 inner. Screws developed from a flat plate hold more water 

 than those having threads at right angles to the surface of the 

 core, and are easier to construct. The effect of the internal 

 frictional resistance with a pump 3 feet in diameter, 10 feet 

 lift, and running at twenty revolutions a minute, he found to 

 vary from ^ to 8 per cent, of the useful effect realised ; the 

 gudgeons being 4 inches diameter absorbed 12 per cent, of the 

 power applied ; and that the best machines give off a useful 

 effect of 85 per cent, of the power applied. For the most 

 economical machine he considered that the spiral angle should 

 not be less than 30° nor greater than 40° ; and that the limit 

 of height at which these pumps can be worked advantageously 

 is ID feet. 



