smaller in diameter, if it is to be driven as rapidly as before. The brass in the rim expands in the heat more than the steel does; thus each half of the rim is bent inward, and the diameter of the balance grows less. When the watch is exposed to cold the hair-spring acquires more vigor, and the watch tends to gain ; but the outer brass portion of the rim contracts more than the inner steel portion, and each half of the ring bends outward, increasing the inertia of the wheel, and thus preventing the gain which would otherwise ensue. a watch. To test the running of a watch one should compare it The rate of with a standard clock every day, or even more often, until a satisfactory knowledge of its performance has been obtained. A watch which is set right today and found nearly correct a month afterward may meanwhile have wandered off two or three minutes, and come back again. FIG. 90.-A WATCH BALANCE. Sometimes a watch exhibits a large daily variation, gaining a considerable fraction of a minute during the first few hours after it is wound, and losing it during the remainder of the day. It is needless to say that a watch must be treated well, if it is expected to do good work. It must not be handled carelessly, nor be permitted to run down; if it runs down, it rarely starts again with the same rate that it had previously. Unless a watch is expensive, and "adjusted for heat, cold, and position," it is likely to Ladies culpable. The regulator. Miscellaneous facts. It exhibit considerable variations of rate, if it is not kept in A young man is likely to move the regulator too often. If his watch suddenly begins to gain a few seconds a day, the regulator is moved backward at once. The less one alters the regulator the better, for a watch, like a human being, is subject to spells of irregularity, from which it recovers if left to itself. If the minute-hand is once set exactly over a minute mark, when the second-hand is at 60, and the two hands do not keep together, either the face is poorly engraved, or the pinion on which the minute-hand turns is not in the center of the face. If either of these hands slipped, which is rarely the case, the same effect would be produced. Occasionally a watch gains a minute or so in an hour; this indicates that the hair-spring is caught, so that it does not vibrate freely; a jeweler will loosen it in a moment. A watch may stop because it has been wound too tightly; a little shaking for a few minutes in such a way as to make the balance wheel vibrate will relieve the difficulty. In general, the possessor of a watch does his full duty by it if he winds it regularly, handles it carefully, keeps it in the same position as much as possible, and has it cleaned once in two years. CHAPTER XI. THE SUN. "See the sun! God's crest upon His azure shield the heavens.” -Bailey. Of all the heavenly bodies the sun is of the greatest importance to man. Without its steady gravitational Its importance. pull on the earth our planet would fly away to unknown regions of space, and the chill of death would settle down upon it. The oceans would stiffen into glass: the rivers would halt in their courses. All the higher forms of vegetable life would wither and die, and humanity, having struggled in vain against inevitable fate, would perish of hunger and cold. For the human race is dependent upon the energy which the sun radiates so lavishly. carbon. The sun stimulates the growing plant to disengage carbon from the embrace of oxygen, feeding on the Oxygen and carbon and leaving the oxygen, which is necessary for the life of men and animals. Its heat evaporates the waters of the oceans, which rise, form clouds, and descend again as rain or dew, quenching the longings of the parched earth, nourishing vegetation, coursing in majestic rivers to the sea, refreshing the bodies of men and animals, and giving delight to all intelligent spirits. The energies of the sunbeams were stored ages ago in primeval forests: the forests were overwhelmed by the mighty deep and buried in a sepulcher of stone. Today men dig up the mummified sunbeams and burn Mummified sunbeams. The staff of life. them in their furnaces and fireplaces. The genial light of the grate fire is due to those ancient sunbeams which are now released from their prison house. The flying locomotive, beneath whose impetuous rush the earth trembles, gets its speed from the sunbeams. The whitehot glow of a Bessemer converter comes primarily from the sun. The water which flows into our houses has been purified by the sun's rays, and has been forced through the pipes by great engines which derive their power from solar energy stored in coal. The electric car is driven by a current generated by a dynamo, and the dynamo in turn by a steam engine which is fed with the sunbeams of bygone ages. The electric light, which turns night into day, is stray sunshine. Nearly all the heavy work of the civilized world is done by the sun. Bread, which is the staff of life, comes from wheat which has been stimulated in its growth by the sunbeams, and moistened by water lifted by the sun. If the mill which reduced the wheat to flour was driven by the wind, we find the source of the wind in heat produced by the sun's rays. If the mill was driven by water power or by steam, we still say that the sun supplied the power which turns the millstones. Even the final process of baking the bread is an application of heat originally derived from the sun. A man's muscles obtain their strength from the food which he has eaten : in the food has been stored the energy of the sun. In fine, we owe to the sun the sustenance of our bodies, the maintenance of our physical energies, the comforts which we enjoy, the cooling breeze, the gentle shower, and the manifold beauties of nature. We proceed therefore to a short study of this wonder-working body, and shall endeavor to gain some notions about its distance, its size, its motion, its changes of appearance, its make-up, its energies, and its future. The distance of the sun from the earth is nearly The sun's 93,000,000 miles. If a straight road were built from the earth to the sun, and the earth, rotating at its present speed, were to start along this highway, like a rolling wheel, more than ten years would elapse before it would reach the sun. For in one day it would travel a distance equal to its own girth, which we will call 25,000 miles. In forty days 1,000,000 miles would. have been left behind; over 3,700 days would therefore be consumed in the entire journey. An express train, traveling fifty miles an hour day and night, without intermission, would require over two centuries to traverse the same distance. distance. How the dis There are many ways of finding the distance of the sun, most of which involve complicated mathematical tance is found. operations. But one of them is easily understood. By a series of beautiful and accurate experiments physicists have measured the velocity of light, which they place at 186,330 miles a second. Astronomers have found that light takes 499 seconds to come from the sun. Therefore the distance of the sun is obtained by multiplying these two numbers together. of the earth from the sun. is not a perfect circle, but an ellipse, its distance from the sun varies. It is nearest to the sun at the beginning of the year; six months later, on July 1, it is almost 3,000,000 miles further away. This is the mean distance When the distance of the sun is known its diameter is easily computed. It is 866,500 miles; this is nearly The diameter. IIO times the earth's diameter. The sun is therefore over 1,300,000 times as large as the earth. If the earth were magnified until it became as large as the sun, and |