Cassini, who adopted Picard's measure of a degree, makes the diameter of the earth 7967 statute miles; others have estimated it at 7917, and some at 7910 miles. But the estimate which is now esteemed most correct, I believe, is, that the equatorial diameter is 7977 English miles, and the polar diameter 7940. From this we may ascertain the solid contents of the earth. The axis of the earth then assumed to be 7940 and 7977 miles respectively, the area of the generating eclipse is (7940 × 7977 × 0,7854=) 49745178,252: and its area multiplied by two thirds of the longer axis, gives the solidity equal to (49745178,252 × 2/3 × 7977=) 264544857944,136 cubic miles. CHAPTER IV. _Arguments in support of Symmes's Theory, drawn from celestial appearances._ That a disposition to hollow cylinders does exist in nature, I think, must be admitted; and that a similar principle exists in the planetary system, at least in some degree, appears to me as certain. Every person has seen or heard of Saturn and his rings. At certain periods of time the appearance of this planet, viewed through a good telescope, represents him to be surrounded with two luminous rings or bodies of matter, concentric with each other, and with the body of the planet. These rings no where adhere to the body of the planet, but are distinct and separate, some considerable distance from him, and from each other, leaving a portion of vacant space between the planet and the rings, through which we see the fixed stars beyond.[4] It is a fact, I believe, admitted by all, and of which we have positive ocular demonstration, that these rings are constituted of some kind of matter, if not solid, at least to all appearance as much so as the body of the planet. Their thickness must be very inconsiderable, for when the edge is turned to the eye it is no longer visible, except to the powerful reflecting telescope of Dr. Herschel.――Thus the rings undergo phases according to the position of the planet in his orbit, which prove them to be opaque, like other bodies in the planetary system, and like them shining by reflection. I am not informed what is the precise velocity of the rotary motion of the rings; probably their varying aspect, or some other cause has prevented a correct observation from being made. However, the planet itself revolves on its axis, with an astonishing velocity; and no doubt the rings also, though perhaps with different degrees of velocity.