WATER. 



mcafuring the velocity by floating bodies, and mull apply the 

 wheel ftream-meafure before defcribed ; this will give the pre- 

 cife velocity of the furface at any given fpot, and we (hould 

 choofe that place where the curvature begins. The velocity 

 fo obtained we muft add to the mean velocity, and find the 

 difcharge by multiplying the fum by the area of the aper- 

 ture. 



Example. — Suppofe the depth of the bottom of the aper- 

 ture to be eight inches beneath the line of the furface of 

 the water ; that the width of the aperture is four feet, and 

 that the aperture is in a thin plate with (harp edges. Alfo 

 that the ft ream is found by the wheel to move with a velo- 

 city of thirty feet per minute, at the place where the furface 

 of water begins to deviate from its regular Hope, and to af- 

 fume a curvature. 



Then take the numbers 57.246 from the firft cafe in our 

 laft table, and multiply it by 2.83, which is the fquare root 

 of eight (tiie depth) ; thus 57246 X z.83 = 162 feet/i-r 

 minute, for the mean velocity of the water ; to this add 

 30 feet for the previous motion = 192 feet /ifr minute. The 

 area of the aperture is 8 inches, or .66 feet x 4 feet = 

 2.66 fquare feet. Multiply 192 feet velocity by 2.66, and 

 we have 510.72 cubic feet per minute, for the quantity dif- 

 charged. 



IVater-Gauge for meafurtng the Quantity of Water afforded 



by any Spring or Brook The molt accurate and convenient 



method for this purpofe, is to conftruft a temporary bank 

 or dam to intercept the ftream, and pen it up into a pond, 

 then in the bank or dam fix a board with an aperture in it 

 for the water to flow through. By meafuring the width and 

 depth of the aperture as before explained, the quantity can 

 be calculated by the rules already given. 



This is what Mr. Smeaton called the water-gauge, and is 



Refuh.'! of Thirteen Experiments on the Difcharge of Wat' 

 tlnough an Aperture open at Top, made by Meflrs. Smeai 

 ton and Brindley, and M. Du Buat. 



i 



The two laft columns of the table are deduced from cal- 

 culation, and agree fo well vrith the obfervations as to give 

 every confidence m the rules. The laft column is calculated 

 on the fuppofition that the aperture is made in a thin plate ; 

 but the laft column but one is according to Dr. Robifon's 

 number, and agrees more nearly with the truth. We believe 



of moft important ufe, to afcertain the quantity of water that Mr. Smeaton's experiment! were made on an aperture 

 which can be procured to fupply a canal, or for a town, or in a board one inch thick ; the aperture was fix inches wide, 

 a mill, or any other purpofe : it is the neceffary prelude for M. Du Buat's four experiments, denoted by * m the 

 undertaking any fuch kind of work, and all perfons em- table, were in an aperture 184 inches wide, which we 

 ployed in fuch purfuits, fliould underftand the manner of have reduced to fix inches, in order to compare them 

 fixing up a gauge, and making the neceffary obfervations. with Mr. Smeaton's. In making this comparifon, we 

 The dam mult be of fuch a height as to pen up the water have not rejefted any experiment, as we were obliged to 



' do in the cafe of difcharge through the apertures beneath 

 the furface. 



Self-regifier'mg Water-Gauge. — When the produce of a fpring 

 or ftream is required with great accuracy, the depth of the wa- 

 ter flowing through the gauge muft be taken very frequently 



into a tolerable large pond compared with the aperture, fo 

 that the furface of water Ihall have no fenfible inclinatio:i or 

 run towards the aperture ; and to avoid this, the larger the 

 pond is the better. The water muft have fo much fall 

 down from the aperture, as to flow away in a clear ftream 



perfeftly free from all obftruiSion of the water below; but during a whole feafon, and a mean of all the refults obtained, 

 it fhould not fpout out fo as to fall far in the air. This would require the conftant attendance of an intelhgent 

 The aperture fhould be a reiEtangular notch cut in the perfon, and would be hable to miftakes ; but a fmall ma- 

 edge of a broad plank ; it will be beft to make the length of chine may be made to fhew the depth by infpeftion, fo that 

 the notch fome even number of inches, as 6, 8, 12, or 24, any careful perfon can keep the account. Thus, at the 

 and the depth correfpondent to the quantity expeifted to fide of the water-gauge, fix up a wooden or tin cyhnder or 



flow through the aperture. 



We recommend that the edges of the aperture be cut 

 fharp, or even faced with a flip of metal plate, and then 

 our firft rule in the laft table will apply with great accu- 

 racy. The more common praftice is, to ufe a plank of one 

 inch thick, and leave the edges of the aperture of that 

 thicknefs, only rounding off" the fharp angles : in this cafe, 

 the fecond theorem in our table muft be ufed; but this is lefs 

 certain, becaufe the lofs of motion from refiftance will not 

 bear a conftant portion in different depths, for the thicknefs 

 of the plank is a conftant quantity, and therefore bears a 

 different proportion to the quantity difcharged, in every 

 cafe of a different depth. 



The accuracy of our rules, when applied to water-gauges, 

 will appear from the following table. 



trunk, which is open at the bottom, fo that the water can en- 

 ter freely. In this trunk, or tube, let a float be placed, having 

 a fmall light rod attached to the float that will rife up from 

 it, and appear above the top of the trunk ; this part muft 

 be divided into inches and tenths, and muft have fome 

 index fixed to the trunk to read the divifions by. This 

 apparatus muft be carefully adjufted ; in the firft inftance, by 

 the perfon who fixes the gauge, fo that its divifions will corre- 

 fpond with the depth of water meafured very exactly in the 

 way we have direded ; then the float will ever after rife and 

 fall with the furface of the water, and will fhew the depth 

 without any neceflity of referring to the original mode of 

 meafurement, unlefs it be to verify the adjuftment. It is 

 obvious that fuch an apparatus muft be fixed fo, that it can- 

 not be deranged either by deCgn or accident. The tube 

 2 i:. 



