78. =The Tides.=--One effect of the attraction between the earth and the moon is quite familiar. I refer to the tides. When the tide rises it is because the water of the ocean feels the attracting force of the moon. The moon actually lifts the water toward itself. The attraction of the sun sometimes increases and sometimes diminishes the tides, according to its position in relation to the moon and the earth. If the land were as movable as the water, or, in other words, if its particles were held together by no stronger attraction than those of water, there would be the same motion that there is in the ocean over the surface of the earth, as in its revolution successive portions of it present themselves toward the moon. 79. =Meaning of the Word Gravitation.=--The attraction thus existing between different bodies of matter separated from each other is called the attraction of gravitation or gravity, in distinction from the attraction of _cohesion_ treated of in the previous chapter. This name was given to it because we have such common examples of its influence in the fall of bodies to the earth. They are said to _gravitate_ toward the earth. And they are said to do so by the force or attraction of gravitation or gravity. The term _terrestrial_ gravitation is sometimes used in speaking of the earth's attraction, in distinction from the same thing in operation in other planets. [Illustration: Fig. 22.] [Illustration: Fig. 23.] 80. =Attraction Toward the Earth's Centre.=--All bodies are attracted toward the centre of the earth. This is because the earth is globular, as may be made clear by Fig. 22. Let the circle represent the earth, and _a_ a body attracted by it. The lines drawn from the body to the earth represent the attractive force exerted by the earth upon the body. It is obvious from these that there is as much attraction on the one side of the line drawn from the body to the earth's centre as there is on the other. The attractive force, then, of the earth as a whole is exerted upon the body in the direction of this middle line. It tends to draw it, therefore, toward the centre. If, therefore, a weight be suspended by a string, the line of the string continued would go to the centre of the earth. This being so, it is clear that two weights suspended by two strings do not hang perfectly parallel to each other. The difference is so slight in an ordinary pair of scales that it can not in any way be perceived. But if it were possible to suspend in the heavens a beam so long as to stretch over a large extent of the earth's circumference, as represented in Fig. 23, the scales attached to it would be very far from hanging parallel to each other. Substances suspended in different parts of the globe are hanging in different directions, and those which are hung up by our fellow-men on the opposite side of the earth, are hanging directly toward us. [Illustration: Fig. 24.]