33 PHYSICS. 
sawing wood, the blade of the saw be held by the wood, the force required 
to overcome this friction, although it has the effect’ of heating the saw, is 
lost, because the object is not to heat the saw, but to cut the wood. To — 
prevent this friction, the teeth of the saw are set outward to each side 
alternately, so as to make an opening wide enough to allow the blade to work 
freely ; and sometimes a piece of wood is inserted in the cut, to keep the 
sides apart. When the friction cannot be altogether prevented, it is eased 
by rubbing the saw with grease. For the same reason, the axles of wheels 
of carts, railway carriages, and other machines, are kept carefully greased. 
(2) By Percussion—On picking up a lead bullet, or rather the flattened 
fragment of one, just after it has struck a metal target, it is felt to be hot. 
The heat of the flattened ball is the exact equivalent of the force with 
which the bullet was moving when it struck the target, together with that 
communicated to the spot struck. Again, when a piece of cold iron is 
hammered, it becomes hot—that is, the energy expended in the blows is 
converted into heat in the iron, just as happens when a button is rubbed. 
(3) By Compression.—When the density of a body is increased by 
compression, heat is developed according as the volume of the body 
becomes diminished. When books are squeezed between the plates of a 
hydraulic press (see fig. 19, page 17), they are found to be heated; in © 
other words, the force applied to the press has been converted into heat. 
Similarly, heat is evolved when a weight is laid on a metal pillar. 
From a consideration of the foregoing and many similar facts, the con- 
clusion has been arrived at, that heat 7s a form of motion. Thus, 
the heat produced by a bullet striking a target is simply the motion, 
which the bullet had before it struck the target, transferred to the atoms 
of the lead as well as to those of the metal struck; and the heat of the 
hammered iron is simply the motion of the hammer transferred to the 
atoms of the iron ; and similarly in any case of friction or compression. 
2. Change of Condition. It must be distinctly understood that all bodies 
_ have a greater or less amount of heat. We are obliged to conceive of a 
point at which there is an entire absence of heat, but of that point we- 
have no experience, and beyond it the heat of bodies differs merely in 
degree. The temperature of the human body is about 90°, and we are 
accustomed to speak of bodies with a lower temperature than this as ¢old, 
and of all having a higher temperature as warm or hot. Taking for granted, 
then, that heat is motion among the atoms of the body, let us consider 
how different bodies are affected by it. In all bodies, the atoms vibrate 
backward and forward, and these vibrations have greater or less velocity 
and extent, according to the amount of heat in the body. The result of 
these vibrations is that the atoms repel each other, so as to make the body 
composed of them, when heated to more than its ordinary temperature, 
occupy a larger space. Iron, for example, expands when heated, as was 
