4 28 DISEASES Class IV. 2. 4. 



rifes on that fide of the earth, which is turned towards the 

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

 raifed at the fame time on the oppofite fide of the revolving 

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

 of the earth, which counteracts the gravitation of bodies near its 

 furface. For the earth and moon may be confidered as two can- 

 non balls of different fizes held together by a chain, and revolv- 

 ing once a month round a common centre of gravity between 

 them, near the earth's furface ; at the fame time that they per- 

 form their annual orbits round the fun. Whence the centrifu- 

 gal force of that fide of the earth, which is fartheft from this cen- 

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

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

 fide of the earth which is neareft it ; to which mould be added, 

 that this centrifugal force not only contributes to diminifh the 

 terrene gravitation of bodies on the earth's furface on that fide 

 furtheft from the centre of motion, but alfo to increafe it on that 

 fide, which is neareft it. 



Another circum (lance, which tends to raife the tide on the part 

 of the earth's furface, which is mod diftant from the moon, is, 

 that the attraction of the moon is lefs on that part of the ocean, 

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

 be fuppofed to attract the water on the fide of the earth neareit 

 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 molt diftant from the moon with a power only 

 equal to one. Hence on the fide of the earth molt diftant from 

 the moon, the moon's attraction is lefs, and the centrifugal force 

 round their common centre of motion is greater ; both which 

 contribute to raife the tides on that fide of the earth. On the 

 fide of the earth neareft the moon, the moon's attraction is fo 

 much greater as to raife the tides ; though the centrifugal force 

 of the furface of the earth round their common centre of mo- 

 tion in fome degree oppofes this effect. 



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

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

 at the two oppofite quadratures ; that is, the greateft gravitation 

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

 lunar day is about fix hours and a half after the fouthing, or af- 

 ter the northing of the moon. 



Circumftances fimilar to thefe, but in a lefs degree, muft oc- 

 cur in refpect to the folar influence on terreftrial bodies ; that 

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

 the earth's furface at noon, when the fun is over them ; and al- 

 fo at midnight from the greater centrifugal force of that fide of 



the 



