(^39) 
fo muck heavier, thaia the Air of the former place wat, whea 
ih^ Mercury ftoodat 292 inches. 
But in making fuch comparifons, we mnft not forget to con- 
fider the Situation of the feveral places, if we mean to make 
Eftimates not only of the weight of the Atmofphere,but of the 
weight aaddenfity of theAir.For,th0Ugh the Scales wil fliewCas 
has been faid ) whether there be a difference of weight in the 
Atmofphere at the two places , yet, if one of them be in a Vale 
or bottoB^, and the other on the cop or feme elevated part of a 
Hill, it is not to be exfpedied, that the Atmofphere, in this 
latter place, fhould gravitate as much/as the Atmofphere in the 
former, on which a longer Pillar of Air does lean or weigh. 
And the mention, I have made of the differing Situation 
of Places, puts me in mind of fomething» that may prove a* 
nother ufe ofour ^^^f^WBarofcope, and which I had thoughts 
of making tryal off^ but was Accidentally hindred from the op- 
portunity of doing it. Namely, that by exactly poyfing the 
Buble at the foot of a high Steeple or Hiil, and carrying it in 
its clofe Frame to the topj one may, by the weight requinte to 
be added to Counterpoife there to bring the Beam to its Hori- 
zontal pofition, obferve the differerxe of the weight of the Air 
at the bottom, and at the top 5 and, in cafe the Hil! be high 
enough^ at fome intermediate Stations. But how far this may 
affiftmen, to eftimate the Ahfdute or Comparative height of 
Mountains, and other elevated Places 5 and what other Ufes 
the Inftrument may be put to, when it is duly improved 5 and 
the Cautions, that may be requifite in the'feveral cafes, that 
fliall be propofed^ I muft leave to more Icafure, and farther 
Confideration. 
7ht Pdrticutars. 
Ofthofe Obfervatms of the Plamt Mars, formerly intimated 
to have been made at London in the Months p/Februaiy 
and Mzrch A. 1661. 
To perform, whatw.is promifed Num, ii,ofthefe Papers, 
pag. 198 ; 'tis thought fit now to publifli the Particular Obfcr- 
vation>3 concerning the fpots in Mars, and their motionjas 
they were made with a ^6 toot Telcfcopc , and produced in 
Kk writing 
