Because the centre of gravity is lower, when he is sitting, than when he is standing; hence, it requires less strength to support the body in a sitting position. Why can a person lie down longer than he can sit or stand? Because the centre of gravity is nearest the earth when he is lying down, and it requires no effort to remain in that position. How do we measure time? We measure time by means of a _pendulum_, such as is used in a common clock. How does the pendulum move? The pendulum moves regularly to and fro, from the line of gravity. Does the pendulum vary any in its vibrations, or the time of moving to and fro? The pendulum does not vary any in its vibrations. [Illustration: Fig. 37.] Why does the pendulum keep moving? When the pendulum is at A, in Fig. 37, gravity causes it to descend to C, and the motion it gains while falling to C carries it up to B; gravity then causes it to descend to C, and the motion it gains while falling to C carries it back to A again. What helps to keep the pendulum in motion? The works inside of the clock help to keep the pendulum in motion. How long must the pendulum be, so as to tick once in every second? The pendulum must be thirty-nine inches long. How does it tick when the pendulum is _shortened_? It ticks more _rapidly_. How does it tick when the pendulum is _lengthened_? It ticks more _slowly_. How then does _cold_ affect a clock? Cold makes the clock go faster by shortening the pendulum. How does _heat_ affect a clock? Heat makes the clock go more slowly by lengthening the pendulum. How may the pendulum always be kept of the same length? By having the weight at the bottom of the pendulum, so that it may be raised or lowered by means of a screw. Specific Gravity. Upon what does the weight of a body, when in the air, depend? The weight of a body depends upon the force with which it is drawn towards the earth. What is this attractive force called? It is called gravitation or gravity. What is gravity? _Gravity_ is the weight of any body. What is specific gravity? _Specific Gravity_ is the weight of any body, compared with an equal bulk of water. Do substances weigh as much in water as they do in air? No; they do not weigh so much. When we drop lead in a vessel full of water, what becomes of the water? The water is forced over the sides of the vessel. How much water is thus forced over the sides of the vessel? A quantity equal in bulk to the lead dropped into the vessel. When the water thus displaced weighs just one pound, how much less will the lead weigh while in the water than it did in the air? It will weigh one pound less. Then how much less is the weight of a body in water than it is in air? Its weight in the water is as much less as the weight of the water which it displaces. When a body has greater specific gravity than water, what will it do? It will sink in the water.