156 POPULAR LECTURES AND ADDRESSES. 



force. The moon attracts any mass at the nearest 

 parts of the earth's surface with greater force than 

 an equal mass near the centre ; and attracts a mass 

 belonging to the remoter parts with less force. 

 Imagine a point where the moon is overhead, and 

 imagine another point on the surface of the earth 

 at the other end of a diameter passing through the 

 first point and the centre of the earth (illustrated 

 by B and A of Fig. 20, p. 161). The moon attracts 

 the nearest point (B) with a force which is greater 

 than that with which it attracts the farther point (A) 

 in the ratio of the square of 59 to the square of 61. 

 Hence the moon's attraction on equal masses at the 

 nearest and farthest points differs by one fifteenth 

 part of her attraction on an equal mass at the 

 earth's centre, or about a 4,32O,oooth, or, roughly, a 

 four-millionth, of the earth's attraction on an equal 

 mass at its surface. Consequently the water tends 

 to protrude towards the moon and from the moon. 

 If the moon and earth were held together by a rigid 

 bar the water would be drawn to the side nearest 

 to the moon drawn to a prodigious height of 

 several hundred feet. But the earth and moon are 



