'4(J2 Meliod of Jrtefmmng the Power of a Stfeam cflVofer. 



The enquiry which moft immediately interefts land-holders, and others who have the ad- 

 vantage of a current of watef, is to afccrtain w hether it will afford fufficient power to juftify 

 the ereclion of a mill, and what that power may be. If the ftream be ample without much 

 fall, it muft necefTarily be applied to move an undcrfliot wheel by its impulfe, and the power 

 will be determinable from the velocity of the water, and the quantity which paffes through 

 the feclion of its bed. One of the eafieft methods of afcertaining thefe data is that given 

 by Dcfaguliers *, as follows. Obferve a place where the banks of the river are deep and 

 parallel, fo as to make a kind of trough for the water to run through, and by taking the 

 depth acrofs make a true feftion of the river. Stretch a firing at right angles over it, 

 and at a fmall diftance another parallel to the firft. Then take an apple, an orange, cr 

 other fmall b-.ll, juft fo much lighter than water as to fwim in it, and throw it into 

 the water above the ftrings. Obferve when it comes under the firfl. firing by means of an 

 half-fecon.l pendulum, a ftop-watch, or any other proper inftrument, and likewife when 

 it arrives at the fecond firing. By this means the velocity of the upper furface, which ia 

 praflicc may generally be taken for that of the whole, will be obtained. The fe(flion of 

 the river at the fecond firing muft be afcertained by taking the depth as before. If this 

 furface or feftion be the fame as the former, it may be taken for the mean feclion ; if not, 

 add both together, and take half the fum for the mean fecTion. The area of the mean 

 fe£lion in fquare feet being then multiplied by the diftance between the ftrings in feet, 

 will give the contents of the water in folid feet, which pafTed from one firing to the other 

 during the time of obfervation. And this, by the rule of three, may be adapted to any 

 other portion of time. Suppofe, for example, the time had been 12", and the hourly ex- 

 penditure of water were required, the proportion would be : As 12" are to 3600', fo is the 

 number of cubic feet obferved to the hourly expenditure in cubic feet. If the mere velo- 

 city be required in proportion to any fixed interval of time, the fame proportion will give 

 it, provided, inftead of the folid contents in the third term, there be taken the difiance 

 between firing and firing. 



The intelligent obferver may in general abridge this operation, by taking notice of the ar- 

 rival of the floating body oppofite two fiations on the fiiore, efpecially when it is not conve- 

 nient to ftrctch a firing acrofs. The arch of a bridge is a good ftation for an experiment of 

 this kind, becaufe it affords a very regular fedion and two fixed points of obfervation : and 

 in fomc inftances the fea pradice of heaving the log may have its advantages. Where a 

 time-piece is not at hand it may fee equally convenient, provided two obfervers attend, to 

 note the time with a half or quarter-f conds pendulum. The half-feconds pendulum is 

 made by fufpending a fmall round button, or other fpherical weight, by a thread looped 

 over a pin of fuch a length that the difiance from the bend of the loop to the centre of 

 the weight fliall be 9,8 inches. 1 he quarter-feconds pendulum is one fourth of this 

 length. If, by obfervations at feverai fiations above and below any particular point of the 

 river, the velocity is not found to vary, the fedlion of the river in all that fpace may be 

 concluded to be uniform ; and it will not be neceffary to determine more than one fedlion 

 by actual meafurement- 



In the cafe of an overflowing pond, or fmall fiream, which will admit of a dam acrofs it, 



* Courfeof Expeiimental Philofophy, ii. 419. 



the 



