On the Difference ofEffeB in Overjhot and Breaft Wfieels. 545 



water with a low fall, for the following reafons : i . The load upon the bucket-wheel pro- 

 duces more friflrion, on the extremities of the axis, than is produced in the bread- 

 wheel by the water, during its paffage down the channel. That this reafoning is good, 

 may be {hewn by a very familiar experiment. Let any wheel be made to reft on its 

 pivots, and then turned round ; it will gradually lofe its velocity, by the friftlon, to a 

 certain point, when it will ftop all at once. The fuddennefs of this ftop will indicate 

 the magnitude of the friftion. If this be fmall, the velocity will much more gradually 

 decay thart in the contrary cafe. On the other hand, to (hew the friftion at the fur- 

 face of a fluid, let a bowl be filled with water, and the fluid be ftirred, fo as to give 

 it an horizontal motion ; it will be found, that this large and heavy mafs, bearing on 

 a furface fo very much exceeding that of the pivots of the wheel, will turn round for 

 a confiderable time, and lofe its motion by very imperceptible gradations : whence it will 

 follow, that the fri£lion is much lefs. 2. In the ufual conftru£lion of overfhot-wheels, 

 part of the fall is loft, in delivering the water, at a certain diftance below the vertex, 

 and part of its a£Hon is loft, by its running out of the buckets, before it has arrived at 

 the loweft point of the revolution. 3. It is a general notion, that large wheels go 

 fteadier, from their operating in the manner of a fly, by means of their inertia; and 

 wheels have been a£tually conftrufted of iron, with a view to this efi'eft. The breaft- 

 wheel is always larger than the overftiot-wheel, under like circumftances ; and it may 

 be conftrufted, under certain limitations, of as large a fize as the engineer may think 

 fit. 4. It is true, that a breaft-wheel, of any fize, might be converted into a bucket- 

 wheel, and carry down the water in its buckets, inftead of its being fuffered to Aide 

 down the ufual channel. But, when we imagine this converfion to take place, we fee; 

 at once, that the aftion would be lefs fteady, from the fucceflive filling of the bucket*, 

 and that the lower portion of the loaded periphery would difcharge fome of its 

 contents much earlier than the point at which the breaft -water ftream would have ccafed 

 to a£i:. 



The numerical computation of the difference of effeft between one wheel and the 

 oAer, according to the dimenfions given in the prefent communication, may be made 

 from the data at page 466 of our prefent volume. It is there deduced, that one 

 horfe will raife two hogftieads and a ha'lf ef water, ^*n feet high, in a minute 5 and this 

 effe£t, from the note at page 465, would require the fall of twice that quantity of water, 

 as a power to overcome friftion, and produce velocity of working. The ftream men- 

 tioned by my correfpondent muft, therefore, afford 415 cubic feet of water per minute, 

 with a fall of 6 feet, if it be equal in power to fix horfes. 



If certain dimenfions be affumed for the buckets of the overfliot-wheel, and an 

 ordinary breaft-wheel be compared with it, the quantity and fall of the wsterj ufelefsly 

 expended on the former, will Ihew the diilerence of effe^. •? 



W.N. 



On 



