CO 
Narrow Labels of Paper, pafied on each fide the Tube^ both 
at the Bottom, and in my Afcent up the Monument. The 
Differences of the Mercury s Height I meafured with a Deci- 
mal Inch Scale on thin Brafs. The Qtiantity of my Afcenr, 
I meafured with a Ganter*s Chain, becaufe a String would 
ftretch. 
By the niceft Obfervation I could m^ke, I found that at 
the height of 82 Feet the Mercury fell y'.- of aa Inch, and at 
about 164 Feet 
. By tarrying above Ibmewhat long, 1 perceived the PreiTure^ 
of the Atmofphere was fomewhac altered, fo that the Mercury 
in my Delcent, was about ^oi of an Inch different from my 
Obfervations in afcending. Upon which, I repeated my Ex- 
periment by afcending and defcending quicker. At both 
which times, my Obfervations agreed exactly with the firiV 
Tryal. From whence I conclude that at every 82 Feet height, 
or thereabouts, the Mercury will deicend j Tenth of ar^ 
Inch. 
I was defirous to have obferved at this time, the Proporti- 
ons of the Dcfcent of the Mercury, according to Dr. WaUus 
Remarks in his Letter to you, l>Jumh, 251. but found it in 
vain on fo (mall a height. However, confidering there is a 
Difference of 8 Feet between Mr. Hallefs Obfervations and 
mine (which would alter the Mrcury pi of an Inch, which 
is perceptible) I am inclined to thiak, that an higher Afceni 
than 82 Feet is neceffary to caufe the Mercury to defcend j%j 
the higher we are in the Atmofphere. But this I leave to 
your, and fuch other better Judgments, and Obfervations. 
Thus, Sir, I have troubled you with the Method I took, 
as wall as the Obfervations themfelves, that you may the bet- 
ter judge how far what I have done deferveth Credit, and al- 
(b, that I may give fome Cautions I found neceffary to be ob- 
ferved in fuch Experiments as thefe. 
To\vhat hath been faid, I beg your Patience, while I add 
aDefcription of a Portable Barometer, which I conceive may 
be of great ufe in the formerj and many other fuch Experi- 
ments. 
Provide a ftrong GlafsTube. Let the Head of it be pinched 
at about an Inch from the top, fo as to make a narrow Neck, 
whofe Orifice lhall be as big almoftasaStraw. This (which 
is Mr. {luare's way) will much bridle the blow of the Mercury 
againft 
