The Philosophical magazine; a journal of theoretical, experimental and applied physics . sidered the weight of m r in my first diagram toogreat, what will they say of the density of u s in my second dia-gram ? In order to nullify the buoyancy, the density ought tobe greater than that of the crust under the continent. But bywhat process could it become denser, if it became at first lighterin becoming solid ? 13. Another means of determining the inferior limit of thethickness of the crust, seems to be suggested by a considerationof the varying pressure the attraction of the moon and sun mustcaus
The Philosophical magazine; a journal of theoretical, experimental and applied physics . sidered the weight of m r in my first diagram toogreat, what will they say of the density of u s in my second dia-gram ? In order to nullify the buoyancy, the density ought tobe greater than that of the crust under the continent. But bywhat process could it become denser, if it became at first lighterin becoming solid ? 13. Another means of determining the inferior limit of thethickness of the crust, seems to be suggested by a considerationof the varying pressure the attraction of the moon and sun mustcause the fluid in the interior of the earth to exert against thecrust. If the crust were very thin, this varying pressure mightcause it to crack. In the following calculation I show that, toresist the force caused by the moon alone, the crust should be atleast 100 miles thick. The force of the sun is to that of themoon as 2 : 5. Hence to counteract their combined effect atnew moon, the thickness should be at least 140 miles. 14. In the following diagram, let E be the centre of the earth. ->:m supposed to be a sphere, consisting of a crust of uniform thick-ness and with fluid within. E A is the radius directed towardsthe moon M. I propose to find the increase of pressure of the 352 Archdeacon Pratt on the Thickness of fluid against a portion of the crust of which C B C is the innersection when the moon is at M, above what it is when the moonis in the prolongation of the radius E F at right angles to E earth and moon are supposed at rest. E the earths mass; M the moons, = yjE; EA = a, 4000 miles; AB = i, CD = ^,EB = 6, c= the distanceof the moon from E = 60«. If p be the density of any point of the fluid; r, s the distancesof that point from E and the moon, then dp E , M , -i- = ^rclr ^ds, p a* s^ by the laws of fluid equilibrium. Now p is variable; but atall points along the inner surface of the crust it is constant, and= J) suppose. Hence integrating along the surface of
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