go Expii-nmiit: and Ohfcrvations on 



two inches, one pound at the end of S will gain an advantage of 7^ times when compared 

 with that at W. Inltcad, therefore, of fixteen pounds upon the table G being equal in 

 cffetl to counterpoife this lall adion, there will be required upwards of 120 pounds. But 

 a man, in this acHon of pumping by a downward preflure, can without difficulty apply hi i 

 whole weight, and with great eafe one third or one fourth of his weight, fuppofe 50 

 pounds. In this cafe the prellurc will be equivalent to fifty times 1 20 pounds, or 6000 pounds, 

 tJiat is to f.iy, nearly tliree tons. 



To compare tiiis engine with a fcrew, in theory, we muft enquire what fincncfs of 

 thread and length of lever would aflbrd a purchafe of i2o to one. Let us fuppofe the 

 thread of a fcrew, fub(litu;ed in the place of the barrel I, to be one tenth of an inch thick ; 

 the diftance from the top of one thread to the top of the next will in this cafe be one fifth 

 of an inch. This is the fpace through wliich the weight muft rife in one revolution. 1 lie 

 power muft therefore move through 120 times that fpace, namely twenty- five inches; but 

 a lever or radius four inches long will dcfcribe a circle fomewhat larger than thi.-, 

 and confcquently fuch an engine would in theory be equal in power to the hydraulic engine 

 we have been contemplating. 



But when the fubjed is viewed praclically, the diflerence between the two engines ap- 

 pears to be very remarkable. All praiSlical men know how very large a part of the 

 force operating by means of engines is employed in overcoming fridions. Every one is 

 aware of the extreme fridion between folids, and the very iliglit friction which takes place 

 between the parts of fluids. This is feen in the common expedient of oiling the pivots of 

 wheels, and in the very gradual decay of motion in fluid bodies ; while folids moving on 

 each other ftop at once, as foon as the force is diminiflied to a certain degree. The fcrew 

 is an organ peculiarly liable to fridion, and this fridion is always much greater than the 

 whole of tlie reading force ; for there are few inflances where a fcrew w ill return from ex- 

 treme prelTure, when the agency upon the lever is withdrawn. It is alfo to be confidered, 

 that the whole force of t!ie weight or refiftance ads diredly upon the face of the fcrew, at 

 which the motion is required to take place. It has not been npprctlated in wliat degree 

 this refinance or fridion increafes with the weight. In lighter adions the fimple ratio has 

 been inferred ; but under more fevere preflures the two metallic faces extrude tlie greater 

 part of the half-fluid matter between them, and appear by the magnitude of their refiftance 

 to be attached to each other by a p.occfs of the nature of cohcfive attradion. For thefe and 

 pther reafons, it appears nearly inipradicable to form any coniparifon between two engines 

 fo different in principle, but fuch as fiiall be deduced from immediate experiment of their 

 eflfcds. 1 am not in pofleflion of numerical data to indicate the adual power of fcrew-engincs 

 orprelTes; which are perhaps the lefs neceffary, becaufe thofe who are the moft ititerefted in 

 the fuccefs of an improvement like the prefcnt, are for the moft pan able to come at tjicfe 

 without difficulty. I intend, befides, to make tliefe the fubjedl of a future communication. 

 The efleds which I obfcrved were thcfe. 



A machine of the new conflrudlon here defcribcd, was employed to prefs fome papers. 

 The force applied to the lever was fo flight, that the inllrument required no faflening to 

 the table on which it flood : but the effed on the upper bar, A B, which was 2\ inches 

 thick, was fuch as bended it out of a ftraight line upwards of a quarter of an inch, and I 

 apprehend tliat it mi^ht have eafily been broken by continuing the pr-cfTure. AViili a fcrew- 



4 prefs. 



