Ixii PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [_Vol. IxXYlii, 



shows that it cannot be attributed to gravitational attraction. 

 It is only, therefore, by a conversion of the record from 

 solar to kmar times that the influences of these other effects 

 can be eliminated, and the gravitational attraction of the moon 

 be detected and estimated, and, for the satisfactory application 

 of this method, it is necessary that the record should cover a 

 complete lunar cycle, or a period of nineteen years. There are 

 only two records extant, and available, which fulfil this requirement, 

 and of these the Italian is not only the most complete and accurate, 

 but is the only one to which the conversion into lunar times has 

 been applied. 



A summary of the figures obtained has been published in our 

 Quarterly Journal,^ and from this we find that in the six hours 

 preceding a meridian passage there were 3337 shocks, and in the 

 six hom-s succeeding only 3270, giving a mean departure, from the 

 general average for six lunar hours, of 33 "5, or almost exactly 1 per 

 cent, of the mean. So small a variation from perfect equality is well 

 within the limit of what might reasonably be expected, if it were 

 purely fortuitous and the stresses set up by the attraction of the 

 moon had no effect whatever. This point will be retm-ned to 

 later on, but it will be useful to see what conclusions may be 

 drawn if the variation is accepted as real, and due to the cause 

 under consideration. The first of these is that the main stress, to 

 which the strain is due, is of a compressive natm-e, consequent on 

 an increase of downward pressure, or a removal of support from 

 below. The second is that the vertical component of the general 

 increase of strain amounts to just 100 times the variation of the 

 corresponding component of the gravitational stress set up by the 

 moon. It has been established by mathematicians that the 

 maximum upward stress set up by the moon, at the points on the 

 surface of the Earth where it is in the zenith or nadir, amounts 

 to 1 '8,450,000 of the Earth's force of gravity, and where it is on 

 the horizon there is a downward stress of just one half of this : 

 the total variation of downward pressure is, therefore, equivalent 

 to a change of about 1/5,630,000 of that due to terrestrial gravi- 

 tation, as between the points and times when the moon is on the 

 horizon, or at the zenith or nadir. But the moon could never be 

 directly overhead in any part of Italy, and a computation of the 

 m.ean range of variation, over the whole period and the whole area 

 concerned, reduces this fraction to about 1;'9,100,000. As the 



1 Vol. Ixxvii (1921) p. 2. 



