108 PROCEEDINGS OF THE AMERICAN ACADEMY 



the chief stresses exist within the compass of the barrel. I may state, 

 that it was my original object to obviate the necessity of stuffing 

 boxes altogether, by using a tinned screw ST. The maximum pres- 

 sure thus obtainable probably exceeds the limit of the above screw 

 compressor. The tinned screw would have a disadvantage in needing 

 to be freshly coated when it became necessary to refill the barrel with 

 oil. Inasmuch, however, as stuffing boxes are thus quite obviated, 

 and as the screw necessary can be cut by very ordinary means in the 

 laboratory, this serviceable device may well be noted. 



23. Finally, I observe that, cceteris paribus, the labor necessary iu 

 producing flie above pressures decreases nearly as the fourth power 

 of the diameter of the screw ; for friction and leverage both increase 

 as the radius, and the resisting pressure as the square of the radius. 

 Similar advantage is gained by increasing the number of threads to 

 the inch. Hence, by supposing the initial pressures to be produced 

 by a thick screw (diameter one inch, say) at one end of the barrel, 

 and the final pressures (above 2,000 atm., when the enclosed liquids 

 have become much more incompressible) produced by a thinner screw 

 (diameter half an inch, say,) at the other end of the barrel, the practical 

 efficiency of the screw compressor would be increased. In such a 

 case, the piezometer tube must be vertical. § 12. However, in 

 limiting my present work to 2,000 atm., I have by no means ex- 

 hausted the power of the above machine. My purpose in doing so 

 was to avoid straining the gauges. I add, in concluding, that among 

 the facilities of the above screw compressor is the almost micrometric 

 accuracy with which pressure can be raised to and maintained at a 

 given value for any reasonable length of time. 



