492 



THE GEOLOGIST. 



Let S, E, and M be the sun, earth, and moon respectively ; and the last in con- 

 junction with the first, as seen from the second. It will be sufficiently accurate for 

 our present purpose to take the distance of the centres of the earth and moon 

 as = 60 radii of the former body, the distance of the centres of the sun and moon 



as = 23,984 times the same unit, and the mass of the sun at 359,551 times that of 

 the earth ; then, putting S and E to represent the attractions of the sun and earth 

 on the moon, respectively, we have 



s : e = 359551 x 60 3 : 1 x 23, 984 2 



- 80899 : 35952 



— 9:4 nearly. 



So that the sun's claim to a visit from a "Lunar Sea" is greater than the earth's 

 in the ratio of 9 to 4 ; and if the moon, according to the hypothesis, were formerly 

 farther from the earth, she would be, by so much, nearer to the sun, when in con- 

 junction ; and hence the attraction of the earth on the moon would be less, and 

 that of the sun greater, at all earlier periods. 



Apologizing for the length of this letter, I am, yours, &c, 



Torquay, Oct. 12th, 1861. William Pengellst. 



„ To the Editor of the Geologist. 



Sir, — Although the subject of the introductory paper of the October number 

 of your justly popular journal more properly belongs to the science of Astronomy 

 than Geology, yet, as some few of your many readers may be led from it to forrn 

 unjust views of a by no means improbable reason which has been assigned for the 

 absence of both air and water in appreciable quantities in that portion of the moon's 

 surface which has ever been subjected to our observation, I think I may be excused 

 for offering a few remarks on this subject, more especially as they may suggest an 

 answer to the query propounded, "Seeing there are waterless ocean (?) cavities on 

 the moon, where have these waters gone to ?" 



It has long been a well- ascertained fact that the moon rotates on her axis, and 

 performs her revolution round the earth in the same period of time ; it is also well 

 known that if a stick loaded with a heavy weight at one end and a light one at the 

 other be swung round by means of a string attached to the centre of this stick, that 

 the heavy end will in the circulation assume a position further from the hand than 

 the light one (see Herschel's Outlines of Astronomy, last edition, chap, vii.), hence 

 it has been suggested by Professor Hansen, that the same cause which makes the 

 heavier end of the stick describe the larger circle may in all probability be the 

 reason why the moon always presents the same, or at least very nearly the same 

 face to our earth, or, in other words, why the time of rotation on her axis and revo- 

 lution round our earth coincide, namely, that in the moon, as in the stick, the centre 

 of gravity does not coincide with the centre of symmetry. Let us now see what 

 effect this would have on the distribution of water and air on the surface of a 

 globe, as, although neither our moon nor the earth are truly circular, the difference 

 of the effect produced in a globe of exactly the same figure as these bodies would be 

 so small as in no way to affect the truth of our deductions or their applications ; 

 then, first, let us take the case of a globe (fig. 1), in which the true centre, or centre 

 of symmetry, and the centre of gravity coincide. In this case supposing the sur- 



