WATER. 



be greatly refilled in its pafiage through fuch contrac- 

 tions ; and this is called by the- workmen wire-drawing the 

 water. 



The velocity of the water in the conduit-pipe, and in its 

 paffage through every valve, will be greater or lefs than the 

 velocity of the pifton, in the fame proportion that the area of 

 the pifton or working barrel is greater or lefs than the area 

 of the paffage of the valve. For whatever quantity of 

 water paffes through any feftion of the working barrel in a 

 fecond, the fame quantity muft go through any one of the 

 paffages : this enables us to modify the velocity of the water 

 as we pleafe, and we can increafe it to any degree at the 

 place of delivery, by diminifhing the aperture through 

 which it paffes, provided we apply fufficient force to the 

 pifton. This is the cafe in the engine for extinguifhing 

 fires ; but no fuch increafe of velocity muft be fuffered in 

 pumps which are required to raife the greateil quantity of 

 water with a given power ; becaufe the power required to 

 farce the water with a great velocity is very confiderable, 

 and the velocity fo obtained adds nothing to the mechanical 

 effeft which is produced. The refiftance arifes from a two- 

 fold caufe ; viz. the friftion of the water againft the fides 

 of the paffage, and ftill more from the refiftance which 

 water oppofes to any fudden change of figure ; for though 

 water is a ptrfeft fluid, and will readily accommodate itfelf 

 to any change of figure by its own gravity, yet, it requires 

 fome time to make fuch change ; and if we force it to 

 change its figure in lefs tim* than it naturally would, it re- 

 quires mechanical power to do fo, juft the fame as to 



comprefs a mafs of clay, or other i'oft and non-elaftic 

 body. 



In praftice, the velocity with which the pillon of the 

 pump moves, determines the fize of the fmalleft paffage 

 through which the water can pafs without unneceffary re- 

 fiftance. Few pumps move with a greater velocity than 8g 

 or lOO feet per minute ; and we think the area of the nar- 

 roweft paffages and pipes fliould bear fuch a proportion tu 

 the area of the barrel, that the water will never be urged 

 with a greater velocity than three feet per fecond, or i8o 

 feet per minute, if the power required to move the pump is 

 an objeft. In general, this will be accomplifhed by making 

 the area of the fmalleft opening equal to half the area of the 

 barrel ; or if the diameter of the barrel is divided into lo 

 parts, the diameter of the leaft opening (hould be 7 of thofe 

 parts. If the pump moves flower, then the paffages may bear 

 a fmaller proportion. The pumps which have folid piifons 

 are preferable, becaufe the valves can be made of any fize 

 which is defired ; but when a valve is made in the pifton, 

 its fize is neceffarily limited to lefs than we have recom- 

 mended. 



EJiimate of the Strength of Men to raife Water. — Various 

 authors have ftated the mean force of a man fo widely dif- 

 ferent, that the ftudent is perplexed which to choofe. The 

 following table contains feveral of thcfe ftatements, wliich 

 we have reduced to one common denomination ; viz. the 

 number of pounds avoirdupois, or the number of cubic 

 feet of water which a man can raife up in one minute to the 

 height of one foot. 



It is not difficult to account for thefe great differences, 

 when we confider how the mufcular force varies in different 

 individuals, and alfo the power of enduring fatigue. The 

 only means of afcertaining the mean force of a man is to take 

 the fum total of the work executed by a number of men 

 acting for a great length of time. This was repeatedly 

 done by Mr. Smeaton, on a very large fcale, and with fo 

 very little variation, that we can very confidently recom- 

 mend engineers to calculate a man's force at 60 cubic feet, 

 or 3750 lbs., raifed one foot ^cr minute; as this is juft one 

 cubic foot per fecond, it will eafily be fixed in the memory. 

 Defaguliers' eftimate of one hogftiead raifed ten feet high per 



minute, is very frequently ufed, and is 5500 lbs. raifed one 

 foot j!>i/- minute, but it is too great for a mean ; and Defagu- 

 liers himfelf called it the maximum, which no machine can 

 exceed. 



When a machine is to be turned by tlie fi)rce of a man 

 turning a winch or handle, the handle ought not to be 

 longer than from 12 to 16 inches; nor fliould it be calcu- 

 lated to make more than 30 turns per minute ; and when 

 moving with this velocity, it Ihould not require a greater 

 force than 165 lbs. preffure upon the handle ; or a man will 

 Bot be able to move it without greater fatigue than he can 

 endure for a day's work. If the handle is required to move 



flower, 



