340 CONJECTURES concrrnine 
Mr. Maine, in the fame philofophical tranfactions, has 
given us the figure of a fpout that fell at Topfham, near 
Exeter. He has depicted it in the act of ftriking a boat as 
it pafled a creek ; from the bottom of which he has drawn 
a rebound of the whole body of the {pout projeGted from 
it toa large diftance ; evidently proving the defcent: And 
which, while he is arguing for the afcent, it would have 
much become him to have accounted for, and to have fhown 
how it agreed with the doctrine of afcent. The fpout 
_proceeding paffed on to the land, and brake off the limbs 
of a tree, beat the thatch off of a houfe, and did perhaps 
various other damage ; but we hear nothing of its carry= 
ing up any of the light fubftances and dropping them at 
great diftances, far from any environs of the place, which 
it would moft certainly have done had there been a whirl- 
wind, or any fupernal fuction employed in the operation. 
The fupernal fuétion which fome have mentioned I 
fuppofe I may pafs over without more than the bare 
mention of it, but whirlwinds we know there are fre- 
quently, and fome of confiderable ftrength; fo that it be+ 
ing the general opinion that fpouts are formed by them, 
it may not be amifs to examine a little what force they 
may reafonably be allowed to have, and the limits of it, 
Their genuine caufe, fuppofing them to be natural pro- 
ductions, is no other than the afcent of the heated and 
confequently lighter air, at the furface, into, or through 
the colder and confequently heavier regions of the atmof- 
phere above: And in proportion to the different degrees 
of heat in one of thefe, and cold in the other, seer the 
ftrength of thefe be; but no more. 
Dr. Arbuthnot, in his treatife on the air, tells us, that 
the rarification of the air in the hotteft day in fummer 
renders it but one-tenth lighter than that of the coldeft in 
winter, or in words to this purpofe, if 1 remember right, 
for I have not his book by me. Suppofing then the up~ 
per region the fame at all times as the lower one in win- 
ter 
