WATER. 



'I 



from the lower end of the column will acquire the fame 

 velocity of motion, from the uniform preflure of the thirty 

 cubic inches which are above it, that one cubic inch let fall 

 from the top would acquire in falling down to the level of 

 the aperture, t/z. fuch a velocity as in a contraiy direction 

 would throw or projeft it to the level from whence it fell, 

 the weights and velocities in both thefe cafes being equal, the 

 products, or wliat we have called mechanical powers, will 

 alfo be equal. We might therefore be led to fuppofe, that 

 a cubic inch of water, let fall through a fpace of thirty 

 inches, fo as to inpinge upon a folid body, would be capa- 

 ble of communicating thereto an equal motion or mechanical 

 effeft by collifion, as if the fame cubic inch had defcended 

 through the fame fpace with a flower motion, and produced 

 the effeft gradually ; for in both cafes gravity acts upon an 

 equal quantity of matter through an equal fpace. 



It is true that the gravitating force afts a longer Ipace of 

 time upon the body that defcends flowly, than upon the 

 other which falls quickly ; but this cannot occafion the dif- 

 ference in the effeft : for we find by experiment, thrt an 

 elaffic body falling through any given fpace will, by coU;fion 

 upon another elaftic body which is fixed, rebound nearly to 

 the height from which it fell : or, by communicating its 

 motion to a body equal to itfelf, will caufe that body to 

 afcend to the fame height. On thefe principles we might 

 conclude, as fome authors have done, that whatever was the 

 ratio between the power and effeft in underfliot wheels, the 

 fame would hold true in overfhot, and indeed in all others. 



However conclufive thio reai'oning may feem, it will ap- 

 pear, in the courfe of the following deduftions, that the 

 effeft of the gravity of defcending bodies is very different 

 from the effeft of the ttroke, of fuch as are nnn-elajlh, 

 though generated by an equal mechanical power. 



It is true that, in the cafes we have abo-ve fnppofed, the 

 power of the fall of water is the fame ; but tiie problem 

 propofed to the engineer is, to obtain from it all or as 

 much as poffible of the power, and render it applicable to 

 fome ufeful purpofe. We have already given our definition 

 of poiuer, that it is weight or matter compounded with 

 motion. Now to obtain all the power from any ftream of 

 water, we muft abftraft from it all its weight and all its 

 motion. In underfhot wheels, or any others moved by the 

 impulfe of the water, we cannot come near this, becaufe 

 we have already Ihewn, that the greatell effeft is produced, 

 when the velocity of the wheel is two-fifths of the velocity 

 of the moving water. The water, after it has finifhed its 

 effeft, is difcharged with that velocity ; hence it retains 

 and carries away with it three-fifths of its original power. 

 Neither can we obtain the full effeft of the weight of the 

 water, for another lofs is fuftained, in the change of figure 

 which the water experiences, when it ftrikes the float-board. 

 This is much greater than is ufually fuppofed, in confidering 

 machines, although it muft be familiar to any one who con- 

 fiders the refiftance of a boat, or other body, when drawn 

 through water. No weight is raifed in thefe cafes, unlefs 

 the motion be rapid, ( fo as to raife a wave before the moving 

 body ; ) but all the power is expended in changing the figure 

 of the water, by dividing the particles, and putting them 

 in new pofitions, fo that the body can pafs between them. 



It is to this fource that we muft look, for the difference 

 between two-fifths of the power, which we find is abftrafted 

 from the whole power of the water by an underfliot-wheel, 

 and one-third of the power, which is the utmoft we can ob- 

 tain by means of an underftiot wheel. 



In the other clafs of machines, which are aftuatcd by the 

 weight of water, we can obtain a much greater fhare of the 

 power of the defcending water. The weight of the water 



is borne by the machine, which muft therefore receive the 

 whole weight of the water, and the lofs is chiefly in the 

 motion which the water itiU retains after departing from or 

 quitting the machine ; but as we are not confined, as in the 

 former inftnnce, to any fixed velocity of motion for the' 

 wheel, we may make it move almoft as flowly as we pleafe,' 

 fo that the water will carry away with it a very fmall fliare' 

 of the velocity which it would have acquired by falling 

 through the height of the fall. Indeed, if v/e could fup- 

 pofe a wheel to be without friftion, and no water to leak, 

 or efcape from thofe veifels, or parts of the wheel which 

 contain the water, it would be poflible to obtain an effeft 

 from it Tery nearly equal to the power. 



Briajl-Wheels. — Thefe are very commonly called under- 

 fliot wheels, becaufe the water runs beneath the wheel, bu: 

 improperly, becaufe the water does not flioot againft the 

 floats of the wheel, or at leaft the principal power is derived 

 from the weight of the water. A breaft-wheel partakes of 

 the nature of both an overfhot and an underfliot, and is con- 

 ftrufted as is reprefented in^^g^. I. PIa!e I. of IValer-ivheeh. 

 The lower part of the wheel is furrounded by a curved wall 

 or fweep of mafonrv, v\-hich is made concentric with the 

 wheel, and the float-boards of the wheel are exaftly adapted 

 to the niaionry, fo as to pafs as near as poffible thereto with- 

 out touching it ; and the fide walls are in like manner 

 adapted to the end of the float -board or fides of the wheel, 

 the intention being, that as little water as poffible fhall be 

 able to pafs by the float-boards without caufing the boards 

 to move before it. The water is poured upon the wheel 

 over the top of the breaiting at I, the efflux from the mill- 

 dam R being regulated by the fluice or fluittle M, which is 

 placed in the direftion of a tangent to the wheel, and is pro- 

 vided with a rack N, and pinion P, by which it can be 

 drawn up fo as to make any required degree of opening, and 

 admit more or leis wster to flow on the wheel. 



The water firll ftrikes on the float, and urges it by its 

 impulfe ; but when tlie floats defcend into the fweep, tliey 

 form as it were clofe buckets, each of which will contain :. 

 given quantity of water, and the water cannot efcape from 

 thefe buckets except the wheel moves, at leaft this is the in- 

 tention, and the wheel is fitted as clofe as it can be to the 

 race with that view. Each of the portions of water con- 

 tained in thefe fpaces bears partly upon the wall of the 

 fweep, and partly upon the floats of the wheel ; and its pref- 

 fure upon the floats, if not exceeded by the refillance, will 

 caufe the wheel to move ; hence the acf ion upon all the 

 floats which are within the fweep of the breafting is br the 

 weight of the water alone ; but the water is made to im- 

 pinge upon the firft float-board with fome velocity, becaufe 

 the furface of the water in the dam K is raifed confiderab!-- 

 above the orifice beneath the fhuitle where the wat; ■ 

 iffues. 



The upper part of the fall at I is rounded off to a fe:^- 

 ment of a circle called the crown of the fall, and the water 

 nuis over it. The lower edge of the fhuttle when put do" n 

 is made to fit to this curve, fo as to make a tight joint ; r.:.d 

 in confequence when the fhuttle is drawn up, the water will 

 run between its lower edge and the crown in a fheet cr 

 ftream which ftrikes upon the firft float that prefents itfelf, 

 nearly in a direftion perpendicular to the plane of the float- 

 board, or of a tangent to the wheel. The float-boards of 

 the wheel are direfted to the centre, but there are other 

 boards placed obliquely which extend from one float-board 

 to the rim of the wheel, and nearly fill the fpace between 

 one float-board and the next. Thefe are called rifing-boards, 

 and the ufe of them is to prevent the water flojwing over the 

 float-board into the interior of the wheel j but the edges of 



thefe 



