(532 FHILOSOPHICAL TRANSACTIONS. [aNNO I694. 



bustible matter, which still continues, wholly ceased, and have never been felt 

 there since; wherefore many expect some such eruption in some of the moun- 

 tains here, though we hope there is no necessity for it; the shocks having been 

 observed to lose their force, and to become weaker and weaker ever since the 

 first fatal one. 



After the great shock, those who escaped got on board the ships in the har- 

 bour, where many continued about two months after; the shocks all that time 

 were violent, and frequent, sometimes two or three in an hour's time, accom- 

 panied with frightful noises, both from under the earth, and from the continual 

 falling and breaking of the mountains. 



Concerning the Distance of tJieJlxed Stars. By Francis Roberts, Esq. S. R. S. 



N°209, p. 101. 



The ancient astronomers, who had no other way of computing the distances 

 of the heavenly bodies, but by their parallax to the semidiameter of the earth; 

 and being never able to discover any in the fixed stars, thence rightly inferred, 

 that their distance was very great, and much exceeding that of the planets; but 

 they could go no farther, but by guess. Since the Pythagorean system of the 

 world has been revived by Copernicus, there seemed ground to imagine that the 

 diameter of the earth's annual course, which is at least 40,00O times larger than 

 the semidiameter of the earth, might give a sensible parallax to the fixed stars, 

 and thereby determine their distance more precisely. 



But there are some considerations which may make us suspect, that even this 

 is not large enough for that purpose. 



M. Huygens tells us, he could never discover any visible magnitude in the 

 fixed stars, though he used glasses which magnify the apparent diameter above 

 100 times. Now, since in all likelihood the fixed stars are suns, perhaps of 

 different magnitudes, we may, as a reasonable medium, persume they are 

 generally about the size of our sun. Let us then, for example, suppose the 

 Dog-Star to be so: the distance from us to the sun being about 100 times the 

 sun's diameter, as is demonstrable from the sun's diameter being 32 minutes, it 

 is evident that the angle, under which the Dog-Star is seen in Mr. Huygens's 

 telescope, must be near the same with the angle of its parallax to the sun's 

 distance, or semidiameter of the earth's annual course; so that the parallax to 

 the whole diameter can be but double such a quantity, as even to Mr. Huygens's 

 nice observation is altogether insensible. The distance therefore of the fixe4 

 stars seems hardly within the reach of any of our methods to determine; but 



