( ^ 0 
I. Part of a Letter of Mr. William Derham, 
SOME time fince, I had occafion to fatisfy my Curi- 
ofity, concerning the Variation of the Mercury on 
different heights ; and accordingly in Sept, 16^6. I 
obferved it on the Monument, by Two of Mr. Quares 
beft Portable Barometers. By the belt of the Two (for both 
differed) i found the Mercury defcended of an Inch at the 
heighth of 80 Feet, and jo at 160 Feet. 
But fince that, finding my Obfervations a little different 
from Mr. Hallefs on Snowden-Hill^ in Vhikf. Tranfa^, Numb. 
229. I thought it neceffary to renew my Experiments more 
nicely ; and accordingly laft 'No'vemher^ tried again with o- 
ther portable Barometers ; which err'd intoUerably. So 
that by the by^ few of the portable Barometers that I have 
yet met with, are to be depended upon in fuch Experi- 
ments. 
I therefore contrived to carry up the Torricellian Experi- 
ment to the Top of the Monument^ thus : I provided a pret- 
ty large Ghfs Tube well cleaned : This I lodged in Wire, 
and filled with well (trained Mercury ; which being cleared of 
all Air, I then plunged the Bottom of the Tube into a broad 
Ciftern of Mercury^ and then fixed both the Tube and Ciftern 
together, in the Wire Cafe or Frame, On the Top I left an 
Eye in the Wire^to fufpend the whole Barometer on a String, 
that it might hang penduloufly, which is abfblutely necelTa- 
ry 5 becaufe if the Ciftern be deeper on one fide than ano- 
ther, or if tlie Tube hang more towards one fide than the o- 
ther, ic will caujfe a great and erroneous Variation in the 
Mercury above, according as the Tube ftands perpendicular- 
ly, or not. 
Mjinflrument being thus (I think) very nicely prepared, 
I marked exadly the Height of the Qaickfilver, upon Two 
Narrow 
