TOL. XVII.] PHILOSOPHICAL TRANSACTIONS. 475 



ought to be observed when his under limb is still above the horizon about 4 of 

 his diameter, or 20 minutes, on account of the refraction, and the height of 

 the eye of the observer above the surface of the sea : or else the amplitudes are 

 to be wrought as the azimuths, reckoning the sun's distance from the zenith 

 90° 36' : this, though it be of little consequence near the equator, will make a 

 great error in high latitudes, where the sun rises and sets obliquely. 



But to return to our hypothesis, in order t© explain the change of the varia- 

 tions, we have adventured to make the earth hollow, and to place another globe 

 within it ; and I doubt not but this will find opposers enough. I know it will 

 be objected, that there is no instance in nature of the like thing ; that if there 

 was such a middle globe it would not keep its place in the centre, but be apt to 

 deviate from it, and might possibly shock against the concave shell, to the ruin 

 or at least endamaging of it ; that the water of the sea would perpetually leak 

 through, unless we suppose the cavity full of water ; that were it possible, yet 

 it does not appear of what use such an inward sphere can be of, being shut up 

 in eternal darkness, and therefore unfit for the production of animals or plants ; 

 with many more objections, according to the fate of all such new pro- 

 positions. 



To these and all other objections that I can foresee, I briefly answer, that 

 the ring environing the globe of Saturn is a notable instance of this kind, as 

 having the same common centre, and moving along with the planet, without 

 sensibly approaching him on one side more than the other. And if this ring 

 were turned on one of its diameters, it would then describe such a concave 

 sphere as I suppose our external one to be. And since the ring in any given 

 position, would in the same manner keep the centre of Saturn in its own, it 

 follows that such a concave sphere may move with another included in it, having 

 the same common centre. Nor can it well be supposed otherwise, considering 

 the nature of gravity ; for should these globes be once adjusted to the same 

 common centre, the gravity of the parts of the concave would press equally 

 towards the centre of the inner ball, which equality must necessarily continue 

 till some external force disturb it, which is not easy to imagine in our case. 

 This perhaps I might more intelligibly express, by saying that the inner globe 

 being posited in the centre of the exterior, must necessarily ascend which way 

 soever it may move ; that is, it must overcome the force of gravity pressing to- 

 wards the common centre, by an impulse it must receive from some outward 

 agent ; but all outward efforts being sufficiently fenced against by the shell that 

 surrounds it, it follows, that this nucleus being once fixed in «^' ? common 

 centre, must always remain there. 



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