HEAT. 37 
a handle, H. To the key are attached three wires: the line-wire, L; a 
wire, E, attached to a Morse’s Recorder, at its own station ; and one, C, 
attached to the copper plate of a galvanic battery. The ordinary position 
of the key is as seen in the figure, the nipple x, in contact with the little 
anvil 6, that end of the lever being held down by the spring, s; so that, 
when a message is to be received, the current passes from L, by A, J, », b, 
and E, to the recorder. But when a message is to be sent to another 
station, the handle, H, is pressed down, the contact between n and bis 
broken, and the nipple m, is now brought down on the anvil a. This 
connects the copper plate of the battery with the line-wire to the distant 
station ; and the current passes from the copper plate through the line- 
wire, through the key at the distant station (which is in the position seen 
in fig. 39), and through the coils of the electro-magnet of the recorder. 
From the recorder it passes down the earth-wire, and then back, through 
the earth, to the zine plate of the battery. The time during which the 
sharp point in the recorder is to press upon the paper, as before described, 
is thus regulated by means of the handle, H. 
HEAT. 
1. Development of Heat.—HratT may be produced by mechanical means 
in three ways, by friction or the resistance of the surfaces of two bodies 
when rubbed, by percussion or the striking of one body against another, 
and by compression. 
(1) By Friction.—If a smooth metal button be stuck on a cork, and 
rubbed on a piece of soft deal-wood, as a form, it will become heated by 
the friction ; and if rubbed long enough, will become so hot as to scorch 
wood and paper, and set fire toa match. Considerable exertion of the arm, 
however, is required to produce the latter result. This experiment affords 
an illustration of a general principle in nature, that all energy expended 
results either in @ certain amount of work done or of heat produced. Accord- 
ingly, energy must be so directed as to produce the exact result desired. 
Tf we wish to produce heat, as in the case of the button, or in warming 
one’s hands, the more energy that is applied to overcome the friction, the 
greater is the amount of heat produced. If sufficient energy be expended, 
the heat becomes so great that the rubbed bodies take fire. Savages, for 
example, light their fires by rubbing two sticks together ; forests have 
been set on fire by the friction of two branches waving in the wind; and 
destructive fires have been occasioned by friction in a piece of machinery, 
More generally, however, energy is directed to the performance of work, 
and in this case all that goes to produce heat is lost. If, when a man is 
