le is constructed as follows: the point corresponding to -460 degrees
Fahrenheit, is taken as the zero point on the new scale, and the degrees
are identical in magnitude with those on the Fahrenheit scale.
Temperatures referred to this new scale are called absolute temperatures
and the point -460 degrees Fahrenheit (= -273 degrees centigrade) is
called the absolute zero. To convert any temperature Fahrenheit to
absolute temperature, add 460 degrees to the temperature on the
Fahrenheit scale: thus 54 degrees Fahrenheit will be 54 + 460 = 514
degrees absolute temperature; 113 degrees Fahrenheit will likewise be
equal to 113 + 460 = 573 degrees absolute temperature. If one pound of
gas is at a temperature of 54 degrees Fahrenheit and another pound is at
a temperature of 114 degrees Fahrenheit the respective volumes at a
given pressure would be in the ratio of 514 to 573.

[Illustration: Ninety-sixth Street Station of the New York Railways Co.,
New York City, Operating 20,000 Horse Power of Babcock & Wilcox Boilers.
This Company and its Allied Companies Operate a Total of 100,000 Horse
Power of Babcock & Wilcox Boilers]

British Thermal Unit--The quantitative measure of heat is the British
thermal unit, ordinarily written B. t. u. This is the quantity of heat
required to raise the temperature of one pound of pure water one degree
at 62 degrees Fahrenheit; that is, from 62 degrees to 63 degrees. In the
metric system this unit is the _calorie_ and is the heat necessary
to raise the temperature of one kilogram of pure water from 15 degrees
to 16 degrees centigrade. These two definitions lead to a discrepancy of
0.03 of 1 per cent, which is insignificant for engineering purposes, and
in the following the B. t. u. is taken with this discrepancy ignored.
The discrepancy is due to the fact that there is a slight difference in
the specific heat of water at 15 degrees centigrade and 62 degrees
Fahrenheit. The two units may be compared thus:

1 Calorie = 3.968 B. t. u. 1 B. t. u. = 0.252 Calories.

_Unit_ _Water_ _Temperature Rise_
1 B. t. u. 1 Pound 1 Degree Fahrenheit
1 Calorie 1 Kilogram 1 Degree centigrade

But 1 kilogram = 2.2046 pounds and 1 degree centigrade = 9/5 degree
Fahrenheit.

Hence 1 calorie = (2.2046 x 9/5) = 3.968 B. t. u.

The heat values in B. t. u. are ordinarily given per pound, and the heat
values in calories per kilogram, in which case the B. t. u. per pound
are approximat