WATER. 



abitratt of every tiling that can be deduced from theory, 

 reCpefting natural and artificial hydraulics. The elegant con- 

 cifenefs of his manner deferves fo much the more praife, as 

 liis countrymen too often make a merit of prolixity. 



In our article Discharge, we have given the general 

 principles of the motion of fpouting fluids ; and under River 

 the theory of water running in rivers. The objetl of the 

 prefent article will be to lay down fuch rules as may be im- 

 mediately applicable to the ufe of the engineer. 



In all cafes of gauging ilreams, the quantity which flows, 

 in any given time, is obtained by meafuring the area of the 

 aperture, or channel, through which the water flows, and 

 Ending the velocity with which the water moves through 

 that aperture. To find the area of the aperture is a fimple 

 operation of menfuration, but to afcertain the velocity is not 

 fo eafy. There are two different methods of determining 

 the velocity. The lirlt is, by obferving the rate of motion 

 of the furface, either by means of fmall light bodies thrown 

 into the ftream, or by employing inllruments adapted to 

 meafure the rate at which the ilream moves. This method 

 i« only applicable in cafes of open canals and rivers, where 

 the water flows with a flow motion. The other method is 

 more general, and is applicable to the greatelt velocities j 

 becaufe it is derived from calculation, according to the depth 

 of water, or height of column, which urges the flowing 

 water, and occafions its motion. 



To meafure the Quantity sf Water running in a River or Ca- 

 nal. Fii-Jl Method. — Chooie a part of the channel where the 

 banks are of a determinate figure, and where they continue 

 of the fame breadth and depth for a length of ten, twenty, 

 or thirty feet, the longer the better, and the more regular 

 the banks are, the better the obfervations will be. Mealure 

 the breadth and the depth, or other dimenfions which may 

 be neceffary, to find the area, or fedion of the paflage, 

 through which the water flows. Take thefe meafures at 

 feveral different points, and if there is any difference at dif- 

 ferent places, find the area at each place, and take a mean 

 between them. 



Then proceed to find the velocity of the motion, by 

 throwing in a cork, or other light body, and obferving, by 

 a llop-watch, or pendulum, what number of feconds it takes 

 to flow through a given length of the channel ; for in- 

 ftance, the length of ten, twenty, or fifty feet, which was 

 chofen in the tirft inflance for the experiment, and marked 

 out by ftretching two firings, parallel acrofs the river. This 

 trial m\ift be repeated feveral times, and as the inilant when 

 the floating body arrives at the lall fl;ring, can be very 

 exaftly noted, this method admits of confiderable exaftnefs. 

 A mean of the different refults muft be taken for the true 

 velocity. 



It is true that this only gives the velocity of the water 

 at the furface, and the water moves with different velo- 

 cities at different depths, beneath the furface; (inilead of 

 a fingle light body to float upon the furface of the water), 

 we are recommended to employ a cylindrical rod of wood, 

 of a length fomething lefs than the depth of the water : 

 this is to be ballafted by a weight at the lower end, fo 

 that it will fwim jufl upright in Handing water, and with 

 the upper end of the flick about an inch above water. By 

 uGng this, inilead of a fingle cork, we are fuppofed to attain 

 the mean velocity of the ftream at its diffe-ent depths, inftead 

 of the velocity of the furface. 



Inftead of a cylinder of wood, three or four apples, 

 ftrung together by a ftring, will anfwer the purpofe very 

 well, the lower ones 'being loaded by putting nails in 

 them till they are rather heavier than water, fo that the 

 apples, when put into ftanding water, will hang in a per- 



VoL. XXXVIII. 



pendicular line. Goofeberries are very nearly the weight of 

 water, and may be employed fingly, to fliew its velocity at 

 different depths. 



Example — A canal meafured eight feet in width, and 

 four feet in deptli, the fides being perpendicular ; then the 

 area of the ftdion is thirty-two fquare feet. It was found, 

 by experiments with three apples, that the current ran 

 through a fpace of fifteen feet in five feconds, in another 

 experiment fix feconds, and in a third four feconds and a half. 

 What is the quantity of water pafling through this canal >. 



The mean of all thefe is five feconds and one-fixth, 

 during which the water moved fifteen "feet. Now as five 

 feconds and one-fixth is to fifteen feet, fo is fixty feconds to 

 a hundred and feventy-four feet, which the ftream flows in 

 the fpace of a minute. Then thirty-two fquare feet (the 

 area), multiplied by 174 feet, gives 5568 cubic feet, which 

 is the quantity of water flowing through the canal every 

 minute. 



This is the method recommended by Defaguliers, and 

 if carefully executed, and the trials frequently repeated, 

 is tolerably exaft. Several autliors have fuppofed this 

 method might be much improved, by employing fome in- 

 ftrument to ftiew the velocity of the ftream by infpeftion. 

 There are many ingenious inventions for this purpofe. 



Stream-Meafurers. — M. Pitot invented a ftream -meafurer 

 of a fimple conftruftion, to find the velocity of any part 

 of a ftream. This inftrument is compofed of two long 

 tubes of glafs open at both ends, and placed in a perpen- 

 dicular direction in tlie Ilream of water : one of thefe 

 tubes is cylindrical throughout and ftraight ; but the 

 other has its loweft extremity bent nearly at right angles, 

 fo as to form a horizontal branch, which gradually en- 

 larges like a funnel, or the mouth of a trumpet ; both 

 thefe tubes are fixed to the fide of a triangular prifm of 

 wood, with the lengths of the tubes parallel to the length 

 of the prifm, and their lower extremities both on the fame 

 level ; the horizontal branch of the tube is carried through 

 the prifm, fo that the end of the trumpet-mouth opens in 

 one of the angles of the prifm. The upright parts of the 

 tubes ftand one befide the other, and are let into grooves 

 in the prifm, fo as to be tolerably well preferved from ac- 

 cidents. The face of the prifm in which thefe tubes 

 ftand, is graduated on the edges clofe by the fides of 

 them into divifions of inches and lines. 



To ufe this inftrument, it is placed perpendicularly in 

 the water in fuch a manner, that the opening of the trum- 

 pet-mouth at the bottom of one of the tubes, fhall be com- 

 pletely oppofed to the direftion of the current, in order 

 that the water may pafs freely through the funnel up into the 

 perpendicular tube. Then by obferving to what height the 

 water rifes in each tube, it will be found to rife higher in 

 the tube with the trumpet-mouth than the other, and the 

 quantity of this difference will be the height due to the Te- 

 locity of the ftream. 



It is manifeft that the water will rife in the ftraight cy- 

 lindrical tube to the fame height as the furface of the ftream : 

 this is by the hydroftatic preffure. But the water of the 

 current entering by the funnel into the other tube, will be 

 compelled to rife above that furface to fome certain height, 

 at which height it will be fuftained by the impulfe of the 

 moving fluid ; that is, the momentum or impulfe of the 

 ftream will be in equilibrio with the column of water fuf- 

 tained in one tube above the 'furface of that in the other. 

 In eftimating the velocity by means of tliis inftrument, we 

 muft have recourfe to the following rules: if the height of 

 the column fuftained by the ftream, or the difference of 

 heights in the two tubes be taken in feet, the velocity ef 

 O the 



