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182. Units of heat; the calorie and the British thermal unit. The calorie is the amount of heat that is required to raise the temperature of 1 gram of water through 1°C'., and the British thermal unit (B. T. U.) is the amount of heat that is required to raise the temperature of 1 pound of water through 1° F. (One B.T.U. = 252 cal.) Thus, when a hundred grams of water has its temperature raised 4° C. we say that four hundred calories of heat have entered the water. Similarly, when a hundred grams of water has its temperature lowered 10° C. we say that a thousand calories have passed out of the water. If, then, we wish to measure, for instance, the amount of heat developed in a lead bullet when it strikes against a target, we have only to let the spent bullet fall into a known weight of water and to measure the number of degrees through which the temperature of the water rises. The product of the number of grams of water by its rise in temperature is, then, by definition, the number of calories of heat which have passed into the water.

It will be noticed that in the above definition we make no assumption whatever as to what heat is. Previous to the nineteenth century physicists generally held it to be an invisible, weightless fluid, the passage of which into or out of a body caused it to grow hot or cold. This view accounts well enough for the heating which a body experiences when it is held in contact with a flame or other hot body, but it has difficulty in explaining the heating produced by rubbing or pounding. Rumford's view accounts easily for this, as we have seen, while it accounts no less easily for the heating of cold bodies by contact with hot ones; for we have only to think of the hotter and therefore more energetic molecules of the hot body as communicating their energy to the molecules of the colder body in much the same way in which a rapidly moving billiard ball transfers part of its kinetic energy to a more slowly moving ball against which it strikes.

Underw cod & Underwood

A modern superdreadnaught of the Tennessee type, the largest now in use, has a displacement of over 32,500 tons,
a horse power of about 33,000, and develops a speed of about 21 knots. Her armor above the water line at her most
vital points is 14 inches thick, while that of the main turrets is 18 inches thick. She carries twelve 14-inch guns which
throw 1400-pound projectiles effectively against an enemy ship 10 or 12 miles distant. The crew numbers about
1000 men and more than 100 officers. One shot from a 14-inch gun possesses the energy equivalent of a volley from
60,000 muskets. The cost of a modern dreadnaught is $22,000,000. At present the largest one is the electrically driven


superdreadnaught Tennessee

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The first nonstop transatlantic airplane flight was made on June 14, 1919, from St. John's, Newfoundland, to Clifden, Ireland, - a distance of 1890 miles. This historic flight-the longest ever made- was accomplished in fifteen hours and fifty-seven minutes, through fog and sleet, at an average speed of 118.5 miles per hour, -a feat which won the $50,000 prize which had been offered for nearly five .years by the London Daily Mail. The plane was driven by two 360-horse-power Rolls-Royce motors and carried 865 gallons of gasoline. It was piloted by Capt. John Alcock and navigated by Lieut. Arthur W. Brown. This airplane had a wing spread of 67 feet and a length of 42 feet 8 inches

183. Joule's experiment on the heat developed by friction. Joule argued that if the heat produced by friction etc. is indeed merely mechanical energy which has been transferred to the molecules of the heated body, then the same number of calories must always be produced by the disappearance of a given amount of mechanical energy. And this must be true, no matter whether the work is expended in overcoming the friction of wood on wood, of iron on iron, in percussion, in compression, or in any other conceivable way. To see whether or not this was so he caused mechanical energy to disappear in as many ways as possible and measured in every case the amount of heat developed.

In his first experiment he caused paddle wheels to rotate in a vessel of water by means of falling weights W (Fig. 168). The amount of work done by gravity upon the weights in causing them to descend through any distance d'was equal


to their weight W times this distance. If the weights descended slowly and uniformly, this work was all expended in overcoming the resistance of the water to the motion of the paddle wheels through it; that is, it was wasted in eddy currents in the water. Joule measured the rise in the temperature of the water and found that the mean of his three best trials gave 427 gram meters as the amount of work required to develop enough heat to raise a gram of water one degree. This value, confirmed by modern experiments, is now generally accepted as correct. He then repeated the experiment, substituting mercury for water, and obtained 425 gram meters as the work necessary to produce a calorie of heat. The difference between these numbers is less than was to have been expected from the unavoidable errors in the observations. He then devised an arrangement in which the heat was developed by the friction of iron on iron, and again obtained 425.

FIG. 168. Joule's first experiment on the mechanical equivalent of heat

184. Heat produced by collision. A Frenchman named Hirn was the first to make a careful determination of the relation between the heat developed by collision and the kinetic energy which disappears. He allowed a steel cylinder to fall through a known height and crush a lead ball by its impact upon it. The amount of heat developed in the lead was measured by observing the rise in temperature of a small amount of water into which the lead was quickly plunged. As the mean of a large number of trials he also found that 425 gram meters of energy disappeared for each calorie of heat that appeared.

185. Heat produced by the compression of a gas. Another way in which Joule measured the relation between heat and work was by compressing a gas and comparing the amount of work done in the compression with the amount of heat developed.

Every bicyclist is aware of the fact that when he inflates his tires the pump grows hot. This is due partly to the friction of the piston against the walls, but chiefly to the fact that the downward motion of the piston is transferred to the molecules which come in contact with it, so that the velocity of these molecules is increased. The principle is precisely the same as that involved in the velocity communicated to a ball by a bat. If the bat is held rigidly fixed and a ball thrown against it, the ball rebounds with a certain velocity; but if the bat is moving rapidly forward to meet the ball, the latter rebounds with a much greater velocity. So the molecules which in their natural motions collide with an advancing piston rebound with greater velocity than they would if they had impinged upon a fixed wall. This increase in the molecular velocity of a gas on compression is so great that when a mass of gas at 0° C. is compressed to one half its volume, the temperature rises to 87° C.

The effect may be strikingly illustrated by the fire syringe (Fig. 169). Let a few drops of carbon bisulphide be placed on a small bit of cotton, dropped to the bottom of the tube 4, and then removed; then let the

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