138 
POPULAR SCIENCE REVIEW. 
tlie advanced stage to which the mechanical operations of the 
engineer have attained, it will he necessary to examine — 
1st. The sources from which we obtain the elements of 
motive power, comprising the steam- engine, its 
varieties, forms, and application. 
2nd. Water-power, as exhibited in water-wheels, turbines, 
and other hydraulic machines. 
3rd. Grinding’, crushing, and cutting machines. 
4th. The machinery for the manufacture of textile fabrics. 
3 th. Locomotive machinery and railway plant. 
Lastly. The machinery of agriculture. 
In following these divisions of our subject, we have for the 
source of all power the heat of the sun, which, according to the 
last new theory of Professor W. Thomson and Mr. Waterton, 
is produced and maintained by a constant shower of meteoric 
matter which falls into that luminary. Assuming this to be true, 
a continuous supply of cosmical heat will produce any descrip- 
tion of motive power, which we convert into mechanical force by 
the combustion of fuel, and by this and other contrivances abate 
the necessity for muscular exertion in man and animals. To 
the same source may be traced the vivifying principle of 
animal and vegetable life; and the deposits of past ages, 
which we are now using as an agent of accumulated force 
or work accomplished, and which constitute, in almost every 
case, the necessary conditions where motive power is required. 
To tlie labours and experiments of Black, Carnot, Mayer, 
Eegnault, and Joule we are indebted for the two principal laws 
of thermo-dynamics, which are, according to the construction of 
Mr. Mallet, thus stated, namely : — 
That heat and mechanical force are reciprocally convertible, and heat on 
Deing evolved, recpiires, or when it disappears, returns in mechanical force, 
an equivalent of 772 foot-pounds for each English thermal unit, viz., for the 
heat of one degree of Fahrenheit’s thermometer in one pound of water. 
In other words, an increment of heat that will raise the tem- 
perature of one pound of water one degree is equal to raising 
772 lb. one foot in height. This is Joule’s equivalent, and 
hag now become general as a measure of work done in foot- 
pounds.* 
Steam and steam-engines are the most important agents 
now in use as a motive power in Great Britain, and from these 
alone we receive nearly the whole of the force that is at present 
* The resistance of one pound evercome through a distance of one foot 
is called a foot-pound. 
