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THE POPULAR SCIENCE MONTHLY. 



and farthest point from the earth. The following table gives the re- 

 sults of the comparison periods of five days being taken in the mid- 

 dle of which occurs a perigee or an apogee of the moon : 



A third means of ascertaining the moon's influence on earthquake 

 phenomena consists in comparing the latter with the lunar day. There 

 are then found to be two maxima corresponding to the moon's passage 

 to the upper and lower meridian, or to what may be called the lunar 

 mid-day and midnight. The minima occur near the middle of these 

 intervals. M. Perrey has made comparisons in this way of 824 shocks 

 felt at Arequipa from 1810 to 1845 ; of the journals of four observers 

 at Monteleone, Messina, at Catanzaro and Scilla, in the years 1783 to 

 1785, which were marked by great eruptions of Vesuvius ; and, lastly, 

 of the journal of M. S. Arcovito, kept at Reggio from 1836 to 1854. 

 There is manifest, more or less markedly, in all these observations, a 

 preponderance in favor of the hours of the moon's passing the me- 

 ridian. 



This constant increase in frequency of the shocks at the times when 

 the tides are strongest would seem to prove that the producing cause 

 extends its action below the earth's surface. The increase is small, it 

 is true, but it is constantly apparent, however the facts may be viewed. 



"We must not lose sight of the local perturbations to which the 

 irregularity of the earth's internal surface may give rise. As M. 

 Perrey has said, the lower side of this shell must consist of curves and 

 anfractuosities, mountains whose summits project into the central 

 liquid like gigantic stalactites, and valleys which approach the outer 

 surface. This internal orographic system must modify the propaga- 

 tion of the subterranean waves. As in narrow and rapid rivers, the 

 waves will be confined, and will gain in power between two moun- 

 tains that obstruct their passage ; they will spread out and lose power 

 in a plain or valley whose configuration allows them to move more 

 freely. Beating against cliffs and other obstructions, they will cause 

 shocks and concussions, fissures, and a partial local falling of the inter- 

 nal vault, the effects of which will be felt at the surface as undulations 

 and tremblings. All these causes combine to make of earthquakes a 

 very complex phenomenon. 



We might expect to find a species of tide-movement in the lava of 

 active volcanoes, but data on this point are lacking. The only fact we 

 have bearing upon it is derived from the observations of Scacchi and 

 Palmieri during the eruption of Vesuvius in May, 1855, who noticed an 



