[ 3 
XXV. Letters of Henry Eeles, con- 
cerfiing the Caufe of the Afcent of Vapour 
and Exhalation^ and thofe of EVinds ; a77d 
of the general Phcenomena of the Weather 
and Barometer. 
Eo the Rev. Tho. Birch, D. D. Secret. R. S. 
L E T T E R I. 
S I R, Lifmore (in Ireland), Nov. 25, 1754. 
Read Jan. 23,^ Received your letter of the 25th of No- 
^ 755 - vember 175-2, in anfwer to my letter 
to the Royal Society concerning the caufe of thunder. 
As you tell me, that they honoured me with their 
defire of my thoughts on fome fubjeds mentioned 
therein, I have venrured to fend you a letter, for 
them, on one of thofe fubjcds, inclofed in two 
■packets, by this pod. I fear, that the Society will 
think my letter too hypothetical 3 our great modern 
philofopher having, in fome meafure, condemned 
the ufe of hypotliefes. But I muft boldly obferve, 
that that objedion is made in a part of his works, 
which is intirely hypothetical : I mean his queries at 
the end of his third book of optics. And I fhall 
venture to diew (if the Society will have patience to 
bear me), that there is a great deal hypothetical in 
his fecond book of optics, where he thinks himfelf 
more afcertained. For he there fpeaks pretty pofi- 
.fively of four caufes of the refradion, &c. of light j, 
three of which muft be wrong; and the fourth (his 
$ther),. 
