( 46 ) 
This Defcription in Words may perhaps be fufficienr, 
but to prevent Obfcurity, I have added a Figure. 
Having propofed the niceft ways I can think of, for 
meafuring the Variation of the Mercury on different 
Heights, there is one Difficulty (partly (uggefted to me 
by a correfpondent Friend) which may bt refolved here- 
by, which I have not yet had Opportunity to make tri- 
al of, and do therefore recommend it to others. And 
that is, Whether the Mercury hath exafl/j the fame ^a- 
riatioHt on one and the fame Height^ at two different 
Times : viz. When the Mercury is hi^\ as in fair Wea- 
ther \ or low, as in foul : I am inclined to think. That 
as the Atmofphere is heavier or lighter, fo its preflure 
being more or left on the fame Height, that confequent- 
ly the Mercury rifeth or falleth fometimes more, fome- 
times lefs on one and the fame Height. As for Inftance, 
If the Mercury finketh of an Inch at the Height of 
82 Feet, when the Mercury ftandethat 30 Inches in the 
Barometer, I Quaere, Whether it will fink fo much when 
the Barometer is at 29 Inches 
Fig, I. reprefenteth the Circle and toothed Ruler, 
without the Interpoficion of the Weather plate; that 
the Contrivance may be the better ften. 
A. A, The Ruler with Teeth, made to Aide up and 
down. 
/.The little Finger that pointeth to ihtMercury's Height 
(which is placed higher on the toothed Rule than it ought 
to be, that it may be feen.) 
f. c. c. c. The Index or Dial-wheel. 
D. D. D. D. The Circle. 
€. e. The Index. 
The 
