Isn’t it wonderful that such a delicate piece of mechanism can be made to run so accurately? And the wonder is increased by the fact that the little machine is, to a great extent, continually moved about, and liable to extreme changes in position and in temperature. Watches of the highest grades are adjusted to five positions as well as to temperature. Some are adjusted to temperature and three positions, and still others to temperature only. The way in which a watch is made to automatically compensate for temperature changes is interesting. Varying degrees of heat and cold always affect a watch. It is a law of nature that all simple metals expand under the influence of heat and therefore contract when affected by cold. Alloys, or mixtures of different metals, act in a similar manner, but in varying degrees. Some combinations of metals possess the quality of relatively great expansibility. Another natural law is that the force required to move a body depends upon its size and weight. So it follows that with only a certain amount of available force a large body cannot be moved as rapidly as a small one. The force of 200 pounds of steam in a locomotive boiler might be sufficient to haul a train of six cars at a speed of thirty miles per hour, but if more cars be added it will result in a slower speed. The same principle applies to a watch as to a railway train. Therefore if the balance wheel becomes larger as it grows warmer, and the force which turns the wheel is not changed, the speed of movement must be reduced. One other natural law which affects the running of watches is this: Variations in temperature affect the elasticity of metals. Now the balance spring of a watch is made from steel, and is carefully tempered in order to obtain its highest elasticity. Increase in temperature therefore introduces three elements of disturbance, all of which act in the same direction of reducing the speed. First, it enlarges the balance wheel; second, it increases the length of the spring; third, it reduces the elasticity of the spring. To overcome these three disturbing factors a very ingenious form of balance has been devised. [Illustration: 170 PARTS COMPOSE A 16 SIZE WATCH MOVEMENT. (A LITTLE MORE THAN 1/2 ACTUAL SIZE)] A watch balance is made with a rim of brass encircling and firmly united to the rim of steel. In order to permit heat to have the desired effect upon this balance, the rim is completely severed at points near each of the arms of the wheel. If we apply heat to this balance the greater expansion of the brass portion of its rim would cause the free ends to curl inward.