III A Letter of Dr. Wallis to Dr. Sloane, 
concerning the Generation of Hail^ at^d of 
thunder and Lightnings and the LffeBs 
thereof. 
Osfordy July i6, 1697. 
SIR, 
I Thank you for the Tranfadions of June^ 
which you fent me; wherein I am well plea- 
fed with Mr Hallys Remarks on the Torricellian Expe- 
riment at the Top of SnowdoH'HiS in Wales, at the 
Height of 1x40 Yards Perpendicular, Where the 
Height of that Quickfilver in the Barofcope was 
3 Inches and If lefs than below at the Sea-fide : 
Which is an Obfervation of good ufe: and would 
have been more fo, had he had the Leifure to make 
like Obfervations at feveral other Perpendicular 
Heights in the Aicent. For from fuch comparative 
Obfervations, we are to make an Eftimate, at what 
Proportion the Height of the Quickfiiver doth de- 
crea(e in reference to the Height of the Place. I mean 
whether in the fame Proportion, or the Duplicate, 
Sub-duplicate, or how otherwife Complicate thereof. 
From whence we may make a Judgment of the 
Height of the Atraofphere, if at leaft it have a deter- 
minate Height. I did once attempt ( a great while 
fince ) a Computation of it ; but wanted a fufficient 
Number of Data to proceed upon. 
But that which is mod furprifing in thofe Tranf- 
aftions is, the prodigious Hails there mentioned; 
Ddddd which 
