COMPARATIVE EXPENDITURE 0F ENERGY. 513
foot-pound, and 33,000 foot-pounds per minute constitute 1 horse-power, the
basis upon which steam-engine power is estimated. Hydraulic power is
specified in terms of the supply of gallons of water per hour at a definite
pressure, and electric power in Board of Trade units. To trace the connec-
tion between the various standards we proceed as follows : —
If H be the head in feet of a column of water and P its pressure per
square foot,
P = wH,
where w is the weight of a cubic foot of water.
Consequently the pressure per square inch is
wH
and, taking w at 62’5 Ibs., H. = 2'307 p.
Hydraulic pressure of x Ibs. per square inch is therefore that due to a
head of 2'307 x feet, and the potential energy of, say, 1,000 gallons at this
head is
1,000 gallons x 10 Ibs. per gallon x 2'307 x feet = 23,070 æ foot-lbs.
Assuming this to be the hourly rate of supply, and noting that 1 horse-
power hour is equivalent to 33,000 x 60 = 1,980,000 foot-lbs., we conclude
that 1,000 gallons of water at x Ibs. pressure possess energy to the extent of
23 070 v
1 980 000 - '01165 x horse-power hours.
The Board of Trade unit of electricity is 1,000 watts per hour. The
watt is the product of 1 volt (the unit of head or pressure) into 1 ampère
(the unit of current), and corresponds in electrical terminology to the foot-
pound of mechanics ; 746 watts are equivalent to 1 electrical horse-power.
Therefore, at the same rate of supply, the Board of Trade unit = ^ = 1'34
horse-power hours.
From the foregoing data the following table is deducible : —
TABLE XXXVII.—Comparison of Power Supply.
One Thousand Gallons of Water under the Following Pressures :— Equivalent to Energy in
Horse-Power Units. Board of Trade Electrical Units.
700 Ibs. per square inch, 8'10 6'09
750 „ 8'74 6'52
800 ,, 9'32 6'95
850 9'90 7'39
900 „ 10'48 7'82
950 „ 11 07 8'25
1000 „ 11'65 8'69
1250 ,, 14'56 10'86
1500 ,, 17'48 13'04
33