SCREW. 



groove e of the wheel D, goes round its axle I, the power 

 of the machine will be as much increafed, as the circum- 

 ference of the groove e exceeds the circumference of the 

 axle ; and if we fiippofe it to be fix times, then one pound 

 at H will balance fix times 48, or 288 pounds hung to the 

 liue on the axle; and hence the power or advantage of 

 this machine will be as 288 to I ; i. e. a man, who by his 

 natural Itrength could lift a hundred weight, will be able to 

 raife 288 hundred, or 14^^ ton weight by this engine. 

 Fergufon's Mech. edit. 4to. p. 44. 



Screw, Archimedes's, or the fpirnl pump, or as it is 

 called in Germany, the ivatcr /nail, is a machine for the 

 raifing of water, firll invented by Archimedes. 



Its ftrudlure and ufe will be underitood by the foUowine 

 defcription of it. A B C D {P/ale XIV. Hydraulic'/, 

 Jig. II.) is a uheel, which is turned round, according to the 

 order of the letters, by the fall of water E F, which need 

 not be more tiian three feet. The axle G of the wheel is 

 elevated fo as to make an angle of about 44°, or between 45° 

 and 60°, with the horizon ; and on the top of that axle is 

 a wheel H, which turns fuch another wheel I of the iame 

 number of teeth ; the axle K of this lalt wheel being parallel 

 to the axle G of the two former wlieels. The axle G is cut 

 into a double-threaded fcrew (asinj^. 12.), exactly refem- 

 bling the fcrew on the axis of the fly of a common jack, 

 which mud be what is called a right-handed fcrew, like the 

 wood fcrews, if the firft wheel turns in the direftion 

 A B C D ; but it mult be a left-handed fcrew, if the ftream 

 turns the wheel the contrary way ; and the fcrew on the axle 

 G mull be cut in a contrary way to that on the axle K, be- 

 caufe thefe axles turn in contrary diredlions. Thefe fcrews 

 'mud be covered clofe over with boards, like thofe of a cy- 

 lindrical cafl< ; and then they will be fpiral tube?. Or, 

 they may be made of tubes of llii? leather, and wrapt round 

 the axle; in (hallow grooves cut therein, as in^. 13. The 

 lower end of the axle G turns conltantly in tne llream that 

 turns the wheel, aiid the lower ends of the fpiral tubes are 

 open into the water. So that, as the wheel and axle are 

 turned round, the water rifcs in the fpiral tubes, and runs 

 out at L through the holes M, N, as they come about 

 below the axle. Thefe holes, of which there may be any 

 number, as four or fix, are in a broad clofe ring on the 

 top of the axle, into which ring the water is delivered from 

 the upper open ends of the fcrew tubes, and falls into the 

 open box N. The lower end of the axle K turns on a 

 gudgeon, in the water in N ; and the fpiral tubes in that 

 axle take up the water from N, and deliver it into another 

 fuch box under the top of K ; on which there may be fuch 

 another wheel as I, to turn a third axle by fuch a wheel upon 

 it. And in this manner water may be raifed to any given 

 height, where there is a ftream fufficicnt for that purpofe to 

 adl on tlie brond float-boards of the firit wheel. Fergufon's 

 Mechanics, Supplement, p. 22. 



An inllrument of a fimilnr nature is called by the Ger- 

 mans a water fcrew ; it confills of a cylinder with its fpiral 

 projeftions detached from the external cylinder or coating, 

 within wliicli it revolves. This machine might not impro- 

 perly be confidered as a pump, but its operation is prccifely 

 fimilarto that of the fcrew of Arcliimedes, It is evident 

 that fome lofs mull here be occafioned by the want of perfcdf 

 contact between tiie fcrew and Us cover ; in general, at Icsll 

 one-third of the water runs back, and the machine cannot 

 be placed at a greater elevation than 30 ' ; it is alio very eafily 

 clogged by accidental impurities of the water ; yet it has 

 been found to raiie more water than the fcrew of Archi- 

 medes, when the lower ends of both arc immerfed to a con- 

 fiderable depth ; fo that if the height of the furface of the 



water to be raifed were liable to any great variations, the 



water fcrew might be preferable to the fcrew of Archimedes. 

 P/n/« XIV. Hydraulics, Jig. 14. 



When a ipiral pipe; confilting of miany convolutions, ar- 

 ranged either m a fiiigle plane, or in a cylindrical or conical 

 furface, and revolving round a horizontal axiii, is connefted 

 at one end by a water-tight joint with an afcending pipe, while 

 the other cud receives during each revolution nearly equal qua.i- 

 tities of air and water, the macliine is called a fpiral pump. It 

 was invented about 1746, bv Andrew Wirtz, apcwterer at 

 Zurich, and it ii faid to liave been ufed with great fuccefs at 

 Florence and in Ruflia : it has alio been employed in this 

 country by lord Stanhope ; and I have made trial of it (fay« 

 Dr. Young) for raifing water to a height of forty feet. The 

 end of the pipe is furnilhed with a fpoon, containing as much 

 water as will fill half a coil, which enters the pipe a little 

 before the fpoon hris arrived at its higheil fituation, the other 

 half remaining full of air, which communicates the preliure 

 of the column of water to the preceding portion, and in this 

 manner the effctt of nearly all the water in the wheel is 

 united, and becomes equivalent to that of the column of 

 water, or of water mixed with air, in the afcending pipe. 

 The air neared the joint is comprefled into a fpace much 

 fmaller than that which it occupied at its entrance, fo that 

 where the height is confiderable, it becomes advifcable to ad- 

 mit a larger portion of air than would naturally fill half the 

 coil, and this lelfens the quantity of water raifed, but it 

 lell'ens alfo the force required to turn the machine. The joint 

 ought to be conical, in order that it may be tightened when 

 it becomes loofe, and the prefTure ought to be removed from 

 it as much as poflible. The lofs of power, fuppofing the 

 machine well conttrufted, arifcs only from the friftion of 

 the water on the pipe, and the fridbon of the wheel on its 

 axis ; and wlicre a large quantity of water is to be raifed to a 

 moderate height, both of thefe refiilances may be rendered in- 

 confiderable. But when the height is very great, the length 

 of the fpiral mull be much increafed, fo that the weight 

 of the pipe becomes extremely cumberfome, and cauies a 

 great friction on the axi:!, as well as a ilrain on the machinery : 

 thus, for a height of 40 feet. Dr. Young found that the 

 wheel required above 100 feet of a pipe which was three 

 quarters of an incii in diameter ; and more than one half of 

 ilie pipe being always full of water, we have to overcome 

 the fridion of about 80 feet of fuch a pipe, which will re- 

 quire 24 times as much excefs of preffure to produce a given 

 velocity, as if there were no friction. The centrifugal force of 

 the water in the wheel would aUo materially impede its alcent 

 if the velocity were confiderable, fii;ce it would be always 

 poflible to turn it fo rapidly as to throw the whole water 

 back into the fpoon. Tlie machine which Dr. Young had 

 eredted being out of repair, lie thought it more eligible to 

 fubilitute for it a common forcing pump, than to attempt to 

 make any further improvement in it, under circumilances fo 

 unfavourable. But if the wheel with its pipes were entirely 

 made of wood, it might in many cr.fes iuceeed better : or 

 the pipes might be made of tinned cojjper, or even of carthctl- 

 ware, which might be cheaper and liglUer than lead. See 

 Jg. 15. 



The centrifugal force, which is an impediment to the 

 operation of Wirlz's machine!,, has fi)nietinies been employed 

 together witii tlie preilure of the almoiplurc, as an imme- 

 diate agent in raiiir.g water, by means of tiie rotatory 

 pump. This maclune confills of a vertical pipe, cauled to 

 revolve round lis axis, and connected above w lili a horizontal 

 pipe, which is open at one or at both ends, the whole being 

 furninied with proper valves to prevent the cfcapc of the 

 water when the machine it at rell. At foon as the rota- 



tlo» 



