meafuring Heights with the Barometer. 751 
latitudes, climates, or zones of the earth, where the ob- 
fervations were made. On this fu ppo fit ion it was natural 
for the mind to form to itfelf fome general hypothefis, 
which might ferve to account for the appearances ; and 
the firft that prefented itfelf was the following : that the 
atmofphere furrounding our globe might poffibly be 
compofed of particles, whofe fpecific gravities were 
really different; that the lighted: were placed at the 
equator; and that the denfity of the others gradually in- 
creafed from thence towards the poles, where the hea- 
vieft of all had their pofition (a> . 
It is a well known and eflablifhed fact, that in the 
middle latitudes, a North or North-eaft wind conftandy 
raifes the barometer, and generally higher as its con- 
tinuance is longer. The contrary happens when a South 
or South- weft wind blows; for I believe it is commonly 
loweft when the duration and ftrength of the wind front 
(t) It was fuggefted by Dr. george fordyce, that equatorial and Green- 
land air might be brought bottled up, and weighed in this country in air of the 
refpe&ive temperatures, by means of a curious balance whereof he is poffeffed, 
in order to fee whether any difference could be difcovered in their fpecific 
gravities. A thought of the fame kind, but more ealily put to experiment, 
occurred to Lieutenant glenie, of the Royal Artillery, namely, that of 
weighing equatorial and polar fea-water. To this gentleman I am obliged for 
his aflillance in part of the manometrical experiments, as well as in feveral of 
the computations . 
Vol. LX VII. 5 D 
that 
