( 49 > 
In Proportion XXII I, he takes into Confideration 
what Influence the Heat has on the Variations of the 
Barofcope , and denies that it caufes any fenfible 
Changes : However, in the fee ond Scholium of this 
Propofiuon, he explains by it, why the Limits of 
the Variations of the Mercury lefien, as the Places 
are nearer the Propics, 
The third or lajl Chapter is taken up in ac- 
counting for the various Changes of the Weather con- 
ne&ed with, or confequent upon the Rife and Fall 
of the Mercury in the Weather-Glafs. The inge- 
nious Author beginning with the Original and Man- 
ner of forming Vapour?, undertakes to fettle and con- 
firm, upon folid Principles, that which the learned 
and fagacious Dr. Halley had long ago communica- 
ted to the learned World, upon this Argument. 
In Proportion VI, he makes ufe of the Princi- 
ple mentioned before, concerning an elaftic Body, 
that itreftores itfelf to a greater Degree of Expanfion 
than it was in before its tremulous Vibrations; and en- 
deavours, from thence, to ^ explain more particularly 
the Way, that Nature takes in forming and defend- 
ing the Cavities of the vapoury Bubbles, and after- 
wards emiting or detaching them from the Surface of 
a Fluid. 
Propoftion VIII gives an Account of what will 
happen to Vapours, according to this Theory, when 
the furrounding Air is condenfed, or rather com- 
prefled by an external Caufe. Mr. Gerfen affirms, 
that in this Cafe it is not poflible for them to defeend. 
In the Scholium fubjoined, he enlarges upon this Sub- 
ject, and maintains, that the fame will happen if the 
G Ait 
