( 
IL J Letter of Mr. W. Derham, KeBorof Up^ 
mmftcr^ dated ]^n. 13. i 6pl. to Dr.Slomc^ 
about a Contrivance to msafme the Height of 
the Mercury in the Barometer^ by a Circle on 
one of the Weather Plates^ with a Kegtjier of 
the Weather J Sec. for the Year 1 6pj. 
TO my former Obfervations about the Barometer^ 
give me leave to fubjoin a Contrivance for mea- 
furing the Height of the Mercury^ which hath lately oc- 
curred to my Thoughts, and which in fome Cafes is 
more commodious, than that way with a crooked Tube 
(I fpake of) This way is by a Circle on one of the 
Weather-plates, of any convenient Diameter. Divide 
this into 100 Parts (which are to anfwer the 100 Parts 
of an Inch.) The Index which pointech to thefe Parts, 
muft be driven round by a fmall wheel, like the Dial- 
wheel of a Watch. This Wheel muft be driven round 
by a ftrait Piece, like a fmall Ruler, with Teeth on one 
Edge thereof ; every Inch whereof muft contain juft the 
fame Number of Teeth as are in the Dial-wheel before 
mentioned foas that by the thrufting up and down this 
toothed Ruler, you may at every Inch turn round the 
Index once. About the middle of this toothed Ruler^ 
a fine Finger muft be fixed to point exadly to the 
Height of the Mercury, And confequently by raifmg 
or depreiling this Finger to the Height of the Mercury^ 
you very exadly fee, on the Circle, the Parts of an Inch, 
which the Mercury rifeth or falleth in the Tube, by rea- 
fon the Circumference of the Circle is Three or more 
Inches about. 
H This 
