472 PHILOSOPHICAL TRANSACTIONS. [aNNO ]7C0. 



vapour discharges itself at the mouths of volcanos, (as in the case of the eartii- 

 quake at Lima,) it might perliaps be possible for a careful observer to trace the 

 thickness of the several strata from thence to the place where the earthquake 

 took its rise, or at least as far as the shore, if it took its rise from under the 

 sea. If this could be once done in any one instance, and tlie velocity of such 

 an earthquake nicely determined, we might then guess at the depth of the 

 cause in other earthquakes, where we knew their velocity, by taking the depths 

 proportional to those velocities, which probably would answer very nearly. 

 1nd\y, If, in any instance, it should be possible to know how much the motion 

 of any earthquake was retarded by passing under the ocean, the depth of the 

 ocean being known, the depth at which the vapour passed would be also ; for 

 the velocity under the water would be to the velocity if there had been no water 

 in the subduplicate ratio of the weight in the latter case to the weight in the 

 former ; allowing earth to be about '24- times the weight of water, the depth 

 will be readily found. 3dly, Let us conceive the earth to be formed according to 

 the idea before given of it, and that the same strata are at a medium of the same 

 thickness for a very great extent, as well in those places where several of the 

 upper ones are wanting as where they are not. On this supposition we may 

 discover the depth at which the vapour passes, by comparing the several veloci- 

 ties of the same earthquake in places where the thicknesses of the superincum- 

 bent mass are different. 



As the observations relating to the earthquake of Nov. 1st, 1755, are too gross, 

 it would be in vain to attempt, by any of the foregoing methods, to determine 

 with any certainty the depth at which the cause of it lay ; but if Mr. M. 

 might be allowed to form a random guess about it, he would suppose, (on a 

 comparison of all circumstances,) that it could not be much less than a mile, or 

 a mile and half, and he thinks it is probable it did not exceed 3 miles. 



Lf^I. Observations on the Comet of Feb. 1760. By the Abbe De la Caille,* 

 F.R.S. Dated Paris, Feb. 18, I760. p. 635. 



I will venture to send you some of my observations on the present comet, 

 because bad weather may have prevented it from being seen in England. 



* M. De la Caille, a celebrated French astronomer, was born at Rumigny, in 1713, and dieJ at 

 Paris in 1762, at only 49 years of age. He was educated at the College of Lisieux at Paris. 

 Here he became the friend of Cassini de Thury, with whom he was associate.) in projecting the 

 meridian line to pass through France. In 1739 he was appointed professor of mathematics in the 

 Mazarine college } and in 1741 elected a member of the Academy of Sciences. In 1750 he went 

 to the Cape of Good Hope to observe the stars in the southern hemisphere. His writings are very 

 numerous, excellent, and greatly esteemed, especially his Elements of Astronomy. 



