On Metal Microphones in vacuo. 23 
clians therefore converge as the latitude increases ; and the air, 
floioing poleioards, is moving in a bed continually becoming 
narrower and narrower, and is finally forced down to the sur- 
face of the earth long before the pole is reached." 
I do not understand this process of " forcing down." But 
this description of the air racing and crowding towards the 
pole is entirely imaginary. A mass of air, or anything else, 
pressed or impelled northwards in the equatorial regions is 
not in the least tending towards the pole, but does begin to 
move in a great circle slightly inclined to the equator. I am 
no meteorologist ; but I presume that the northward motion is 
reinforced while it still moves in hot regions ; and so the 
great circle in which the motion is supposed to take place may 
become more and more divergent ; and the northward motion 
may be sensible up to the latitudes of 30° or 40° mentioned 
by Mr. Scott. But a general flow of air in the upper regions 
from west to east within a certain tropical belt is all that I can 
infer from the data. 
I haA^e also found that- Professor Everett, in his latest edi- 
tion of the ' Natural Philosophy/ at p. 525 accepts Mr. Ferrel's 
theory of the Conservation of Areas, and at p. 527 refers to 
Mr. Ferrers paper of 1860 as " the most complete exposition 
we have seen of the general atmospheric circulation." 
IY. On Metal Microphones in vacuo. 
By J. Muneo, A.S.T.E* 
THE consideration that a metal microphone would not 
oxidize to any serious extent in a vacuum, and also a 
desire to see how it would operate in exhausted air, led me to 
construct the apparatus illustrated below. 
It consists of a tubular bulb of glass, M, into which the 
* Coinmunicated by the Author. 
