110 PHILOSOPHICAL TKANSACTIONS. [anNO 1750. 



heave, and consequently the earth must heave too. There was a hollow, obscure, 

 rushing noise in the house, which ended in a loud explosion up in the air, like 

 that of a small cannon : the whole duration, from the beginning to the end of 

 the earthquake, seemed to be about 4 seconds of time. The soldiers who were 

 on duty in St. James's Park, and others who were then up, saw a blackish cloud, 

 with considerable lightning, just before the earthquake began ; it was also very 

 calm weather. 



" In the history of earthquakes it is observed, that they generally begin in calm 

 weather, with a black cloud. And when the air is clear, just before an earth 

 quake, yet there are then often signs of plenty of inflammable sulphureous matter 

 in the air ; such as Ignes Fatui or Jack-a-Lantems, and the meteors called 

 falling stars. 



" Now I have shewn many years since, in the appendix to my Statical Essays, 

 experiment 3, page "280, the effect that the mixture of a pure and a sulphureous 

 air have on each other ; viz. by turning the mouth downwards, into a pan of 

 water, of a glass vessel of a capacity sufficient to hold about two quarts, with a 

 neck about 20 inches long, and 1 inches wide ; then, by putting under it, in 

 a proper glass vessel, with a long narrow neck, a mixture of aqua fortis, and 

 powdered pyrites, viz. the stone with which vitriol is made, there will be a brisk 

 ferment, which will fill the glass with redish sulphureous fumes; which, by ge- 

 nerating more air than they destroy, will cause the water, with which the whole 

 neck of the glass vessel was filled, to subside considerably. When the redish 

 sulphureous air in the upper part of the glass is clear, by standing 2 or 3 hours, 

 if then the mouth of the inverted glass be lifted out of the water, so as to let the 

 water in the neck of the glass fall out ; which, supposing it to be a pint, then an 

 equal quantity of fresh air will rush in at the mouth of the neck of the vessel, 

 which must immediately be immersed in the water : and on the mixture of the 

 fresh air with the then clear sulphureous air, there will instantly arise a violent 

 agitation between the two airs, and they will become, from transparent and clear, 

 a reddish turbid fume, of the colour of those vapours which were seen several 

 evenings before the late earthquakes : during which effervescence, a quantity of 

 air, nearly equal to what fresh air was let in, will be destroyed ; which is evident 

 by the rising up of the water in the neck of the glass, almost as high as before. 

 And if, after the effervescence of the mixed airs is over, and become clear again, 

 fresh air be admitted, as before, they will again grow reddish and turbid, and 

 destroy the new admitted air as before ; and this after several repeated admissions 

 of fresh air : but after every readmission of fresh air the quantity destroyed will 

 be less and less, till no more will be destroyed. And it is the same after stand- 

 ing several weeks, provided, in the mean time, too much fresh air had not been 



