WATER. 



penfate for the ftownefs of their motion. The wheel beings 

 tlius more loaded with water, the ftrefs upon every part of 

 the work will be increafed in proportion. 



The beft rule for praftice will be, to make the velocity 

 of the circumference a little more than three feet in a 

 fecond. 



Experience confirms, that this velocity of three feet in a 

 fecond, is applicable to the greateft overlhot wheels as well 

 as the fmalleft ; and all other parts of the work being pro- 

 perly adapted to this velocity, the fall of a given quantity 

 of water, will produce very nearly the greateft effeft pofli- 

 ble. But it is alfo certain from experience, that large 

 wheels may deviate further from this rule before they will 

 lofe their power, by a given aliquot part of the whole, tlian 

 fmall ones can be admitted to do ; for inftance, a wheel of 

 twenty-four feet high may move at the rate of fix feet per 

 fecond, without loiing any confiderable part of its power. 

 This may perhaps be accounted for, when we confider how 

 fmall a proportion of the whole fall is requilite to give the 

 water the proper velocity which the wheel ought to have ; 

 whilfl in a fmaller wheel, the fame height muft be allowed 

 for that purpofe, and confequently, a greater proportion of 

 the whole height. On the other hand, Mr. Smeaton tells 

 us, that he had fecn a wheel of tliirty-three feet diameter 

 that moved very fteadily and well, with a velocity but little 

 exceeding two feet per fecond. 



There is a natural wifh to. fee a machine move brifldy ; it 

 has the appearance of aftivity : but a very How motion al- 

 ways looks as if the machine was overloaded. For this rea- 

 fon, mill-wrights have always yielded flowly, and with reluc- 

 tance, to the advice of Mr. Smeaton, but tliey liave yielded ; 

 and we now fee them adopting maxims of conilruftion more 

 agreeable to found theory, that is, making their wheels of 

 great breadth, and loading them witli a great deal of work. 

 The reluftance to adopt this fyftem did not arife folely 

 from prejudice, but from a real inconvenience attending 

 the flow motion of the wheel when the refiftance which is 

 oppofed to its motion, and which is the caufe that it 

 moves flowly, is not uniform in tlie different parts of a 

 revolution. 



In all machines, there are fmall inequalities of aftion 

 which are unavoidable ; and in fome machines very great in- 

 equalities arife, from the intermitting motions of cranks, 

 ftaDipers, and other parts which move unequally or reci- 

 procally. When a water-wheel is employed to give motion 

 to fuch machines, it may be fo refitted or loaded, as to be 

 nearly in equilibrio with its work, in the moft favourable 

 pofition of the parts of the machine ; but when thefc change 

 into a lefs favourable pofition, the machine may flop the 

 wheel altogether, or at all events hobble, and work very 

 irregularly. And for tlie fame reafon that a water-wheel 

 accommodates its motion very quickly to the refiftance it 

 is to overcome, fo all tendency to irregular motion is in- 

 creafed. A wheel, when its load is increafed, moves more 

 flowly, and receives more water into each bucket ; thereby 

 taking to itfelf a weight of water equal to overcome its 

 load, and on the other hand by moving quicker, it takes 

 lefs water into each bucket when the load is diminifhed. 

 But thefe changes do not take place inftantaneoufly, be- 

 caufe it can be only in the moment that each bucket paffes 

 beneath the ftream, that the fhare of water it fhall have, 

 will be influenced by the rate of the wheel's motion. 

 When a bucket is once filled it continues with that charge 

 until it arrives at the bottom of the wheel. 



This felf-regulating property of the wheel can only ap- 

 ply in cafes of fmall and permanent changes of refiftance, 



for it always comes too hte to correft fudJen and confider- j 

 able changes in the refiftance ; then it afts in the contrary 

 direftion. Suppofe, for inftance, an overfliot wheel is em- 

 ployed to work a fingle pump by means of a crank, the 

 refiftance of this machine will be continually varying ; it 

 will be nothing during one-half of the period of the revo- 

 lution when the pump is not drawing any water, and during 

 the other half it will be in a conitant ftate of increafe and 

 diminution. Now, during the time this wheel has nothing to 

 do, it will turn round very quickly, and therefore each 

 bucket will receive very little water ; confequently, when 

 the wheel comes to be refifted, the wheel will have fo little 

 water in its buckets, that it will perhaps be quite ftopped: 

 in this cafe, the bucket beneath the fpout will receive water 

 until it is quite fuO, and then the water will run over and 

 fill fo many of the buckets beneath it, as to put the wheel 

 in motion flowly; in confequence, the fucceeding buckets 

 will receive a large fhare of water during the half revolu- 

 tion when the pump makes its ftroke ; but when this is 

 finiflied, and the refiftance ceafes, the wheel being well 

 loaded with water, will in confequence move very rapidly 

 for a half revolution, and its buckets will receive very little 

 water. 



This is indeed an extreme cafe of irregular refiftance, 

 and muft be remedied by applying two pumps inftead of 

 one, or a balance-weight, or a fly-wheel ; but the fame 

 principle will apply in cafe of fmaller irregularities. In all 

 cafes, the refiftance muft be reduced to a great degree of 

 uniformity, before a water-wheel can be applied to it with 

 advantage, particularly if the wheel is intended to move 

 flowly, with a view of obtaining the greateft power, the 

 irregularities will then have more ferious confequences. 



A little more velocity enables the machine to overcome 

 thofe increafed refiftances by its inertia, or the great quan- 

 tity of motion inherent in it. Great machines pofTcfs this 

 advantage in a fuperior degree, and will confequently work 

 fteadily with a fmaller velocity. In all cafes, the machine 

 muft have fo much moving matter in it as is fufficient to 

 overcome the irregularities, and regulate tlie motion of the 

 wheel. If this is not already found in the machine, as in the 

 mill-ftones of a corn-mill for inftance, the weight muft be 

 placed in the water-wheel itfelf, or in a fly-wheel appUed 

 for the purpofe. 



Mr. Buchanan meafured the quantity of water which a 

 cotton-mill required, when going at its common velocity ; 

 and when going at half that velocity. The refult was, that 

 the laft required juft half the quantity of water wliich the 

 firft did. In the experiments, the quantities of water were 

 calculated from the depth of water and apertures of the 

 fluices. 



From which experiments, he inferred that the quantity 

 of water neceffary to be employed in giving different de- 

 grees of velocity to a cotton-mill, muft be nearly as the 

 velocity. The water from the cotton-mill on which he made 

 the obfervation, falls a little below it, into a perpendicular- 

 fided pond, which ferves as a dam for a corn-mill. By mea- 

 furing the time which the water took to rife at a certain 

 height in that pond, he determined the expenditure of water 

 when the corn-mill moved at its common velocity ; and alfo 

 when it moved at nearly half that velocity. 



The refult of thefe experiments approached very nearly 

 to the former, and all the differences could be accounted for, 

 by a fmall degree of leakage, which took place at the 

 fluices on the lower end of the pond ; and the time being 

 greater when the mill moved flower, the leakage would of 

 courfe be greater. 



In 



