428 DISEASES CLASS IV. 2. 4. 



rises on that side of the earth, which is turned towards the 

 moon; and follows it, as the earth revolves. Another tide is 

 raised at the same time on the opposite side of the revolving 

 earth, which is owing to the greater centrifugal motion of that 

 side of the earth, which counteracts the gravitation of bodies 

 near its surface. For the earth and moon may be considered as 

 two cannon balls of different sizes held together by a chain, and 

 revolving once a month round a common centre of gravity be- 

 tween them, near the earth's surface; at the same time that they 

 perform their annual orbits round the sun. Whence the cen- 

 trifugal force of that side of the earth, which is farthest from this 

 centre of motion, round which the earth and moon monthly re- 

 volve, is considerably greater, than the centrifugal force of that 

 side of the earth which is nearest it; to which should be added, 

 that this centrifugal force not only contributes to diminish the 

 terrene gravitation of bodies on the earth's surface on that side 

 furthest from the centre of motion, but also to increase it on that 

 side, which is nearest it. 



Another circumstance, which tends to raise the tide on the 

 part of the earth's surface, which is most distant from the moon, 

 is, that the attraction of the moon is less on that part of the 

 ocean, than it is on the other parts of the earth. Thus the moon 

 may be supposed to attract the water on the side of the earth 

 nearest it with a power equal to three; and to attract the central 

 parts of the earth with a power equal to two; and the water on 

 the part of the earth most distant from the moon with a power 

 only equal to one. Hence on the side of the earth most distant 

 from the moon, the moon's attraction is less, and the centrifugal 

 force round their common centre of motion is greater; both which 

 contribute to raise the tides on that side of the earth, On the side 

 of the earth nearest the moon, the moon's attraction is so much- 

 greater as to raise the tides; though the centrifugal force of the 

 surface of the earth round their common centre of motion in 

 some degree opposes this effect. 



On these accounts, when the moon is in the zenith or nadir, 

 the gravitation of bodies on the earth's surface will be greatest 

 at the two opposite quadratures; that is, the greatest gravitation 

 of bodies on the earth's surface towards her centre during the 

 lunar day is about six hours and a half after the southing, or after 

 the northing of the moon. 



Circumstances similar to these, but in a less degree, must oc- 

 cur in respect to the solar influence on terrestrial bodies; that 

 is, there must be a diminution of the gravity of bodies near 

 the earth's surface at noon, when the sun is over them; and also 

 at midnight from the greater centrifugal force of that side of 



