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A.—MATHEMATICS AND PHYSICS. 17 
encourage it, was inaugurated. In this country Mr. W. H. Dines did 
Trojan service in the cause, and his observations and deductions are out- 
standing in the mass of data accumulated in many parts of the world. 
Naturally the conditions over Kurope and North America were investi- 
gated in the greatest detail, but every opportunity has been taken by 
meteorologists to obtain upper-air data from all parts of the world. In 
addition to the regular observations undertaken in most countries having 
an organised meteorological service, expeditions have gone out specially 
to investigate the upper atmosphere over the oceans and over tropical 
Africa, and nearly all recent polar expeditions have included this investiga- 
tion amongst their scientific activities. 
There are, of course, large tracts of the earth’s surface above which 
no observations have yet been made, but some, if only a few, observations 
have been made in all meteorologically important areas, including both 
polar regions. It is on the results of these observations that we base our 
conception of the thermal structure of the atmosphere, and meteorologists 
have attempted from them to generalise the conditionsin all parts of the world. 
The most important generalisation of this kind has been made by 
Sir Napier Shaw and published in the form of diagrams in his book, 
_* The Air and its Ways.’ I shall use these diagrams as the basis of the 
following discussion. 
Probably everyone here is familiar with the main results of these investi- 
_ gations. The atmosphere, which itself is an extremely thin film of air, is 
composed of two shells surrounding the earth. Inthe lower of these shells, 
called the troposphere, the temperature decreases as one rises in the 
atmosphere, and the air is warmer over the equator than over the poles 
at corresponding heights. In the upper shell, called the stratosphere, 
_ the temperature conditions are entirely different. There is little or no 
_ change in temperature with height and the horizontal change of temperature 
is reversed, the temperature at the same height in the stratosphere 
decreasing as one passes from the poles to the equator. At the earth’s 
surface the mean annual temperature near the equator is 27° C., and at 
the poles —23° C., z.c. the equator is 50° C. warmer than the poles. At 
twenty kilometres above the surface the temperature over the equator 
is —80° C. and over the poles —30°C. That is, the temperature difference 
_ between the equator and the poles is the same in amount at the surface 
and at a height of twenty kilometres, but in the former case it is the 
equator which is the warmer, while in the latter it is the polar regions—a 
truly remarkable reversal. 
| The surface of separation between the two shells, called by Sir Napier 
Shaw the ‘tropopause,’ is extremely sharp. There is no region of 
transition. The stratosphere sits on the troposphere like a layer of oil 
on a layer of water. The boundary is, however, not horizontal, and, 
therefore, not exactly concentric with the earth’s surface, being higher 
at the equator than at the pole. In other words, the lower atmospheric 
shell, the troposphere, is thicker at the equator than at the poles. At 
the equator it is nearly twenty kilometres thick, while at the poles it 
thins down to a layer less than six kilometres thick in the summer and 
less than four in the winter. 
I have already said that in the troposphere the temperature decreases 
as one ascends. The magnitude of this decrease varies from place to 
1925 q 
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