(I70 
III. Part of a Letter from Mr. Stephen Gray, 
ahoHt a Way of Meafnring the Heighth of the 
Mercury in the Barometer more exaSlly. 
SI N C E I faw Mr. Derbams Contrivance for mea- 
furing the Minute Variations of the Mercury in the 
Barometer, I have had fome Thoughts on that Subjed: 
I efteemed the Way he mentions {Numb. 137. of your 
FhiUfophkal Tranfa^ions) with a toothed Ruler and Cir- 
cle on the Weather-Plate, very ingenious yet I concei- 
ved 'twould be more accurate , if there were added 
fomewhat to affift the Eye in fetting the Index to the 
Surface of the Mercury. This put me upon thinking 
how to adapt a Microfcope to the Barometer, and tho' 
I could not, prefently, propofe to my felf the mofl: con- 
venient way to effeft it; yet upon re-afluming thefe 
Thoughts, I overcame the Difficulty, as you will find 
by the Defcription I herein give of a double Microfcope, 
furnifhed with a Micrometer ; by which the Mercury's 
Variations may be obferved totheThoufandth Part of an 
Inch. 
A(Fig.6.)A longfquareTable towards one End isereded 
a fquare Column, BB. Upon which there Hides a (quare 
Socket. C, From one Side whereof proceeds a crooked 
Arm, D E. At D there is a Screw-Hole to receive the 
Scre^, and at E a Ring to fupport the Tube of the Mi- 
crofcope, F. From the other Side the Socket, comes a 
Short Arm G, having a Screw Hole to receive the long 
Screw I I, whofe length may be about Sixot Seven In- 
ches; its lower End, by a fmall Hole in its Center, refts 
on the EnJ of a fmall Screw, that comes through the 
Screw- Hole, in the Arm H ; which is fixed on the back 
Side of the Column ; the upper End of the Screw is filed 
left 
