“Tyndall, in his work on heat, gives an excellent illustration of the force of expansion and contraction. ‘The choir of Bristol Cathedral was covered with sheet lead, the length of the covering being 60 feet and its depth 19 feet 5 inches. It had been laid in the year 1851, and two years afterward it had moved bodily down for a distance of 18 inches. The descent had been continually going on from the time the lead had been laid down, and an attempt to stop it by driving nails into the rafters had failed, for the force with which the lead had descended was sufficient to draw out the nails. The roof was not a steep one, and the lead could have rested on it forever without sliding down by gravity. What, then, was the cause of the descent? The lead was exposed to the varying temperatures of day and night. During the day the heat imparted to it caused it to expand. Had it lain upon a horizontal surface, it would have expanded all around; but as it lay upon an inclined surface it expanded more freely downward than upward. When, on the contrary, the lead contracted at night its upper edge was drawn more easily downward than its lower edge upward. Its motion was, therefore, exactly like that of a common earthworm; it pushed its lower edge forward during the day and drew its upper edge after it during the night, and thus by degrees it crawled through a space of 18 inches in two years.’ “Mention has been made in a preceding article of the effect of unequal expansion upon two different metals that have been bolted together. It is by this principle that the action of the ordinary thermostat, so familiar now as a controller and regulator of the temperature of high buildings, is explained--a rod made up of two different metals whose rates of expansion are different. When the temperature of the room in which the thermostat is placed becomes too high the rod curls toward the metal point S and touches it, completing an electrical contact which causes a motor to shut off the draft. When the temperature of the room falls below a certain point the rod curls in the opposite direction toward the metal point T. This causes a motor to open the draft and thus furnish a more abundant supply of hot air. “Everybody in these days of cheap and reliable timepieces carries a watch. And yet there are very few who appreciate the methods and devices by means of which the troublesome expansion and contraction of metals are corrected, in order that a watch may keep correct time. The balance wheel of a watch corresponds to the pendulum of a clock, and any variation in its dimensions will cause it to move faster or slower, as the case may be. The hairspring is really a long strip of metal which becomes weakened in its effect when expanded by an increase in temperature and has its power augmented when contraction takes place.