266 Proceedings of the Royal Society of Edinburgh. [Sess. 
Temperature Centigrade 
or t- 273*7. 
Elevation from Earth’s 
surface required to cool 
moist air by 1° C. 
dx 
o 
- dt 
0 
Metres 152 
5 
168 
10 
186 
15 
207 
20 
229 
25 
252 
30 
274 
35 
284 
§ 16. From this we see that an ascending current of moist air at 3° C. 
would sink in temperature at about the rate of 1° C. in 161 metres of ascent. 
This is exactly Welsh’s gradient; “ and we may conclude that at the times 
and places of his observations the lowering of temperature upwards was 
nearly the same as that which air saturated with moisture [at 3° C.] would 
experience in ascending.”* But it is not to be supposed, indeed it cannot 
have been the case, that his observations were made in a single ascent 
through cloud. “ It is to be remarked that except when the air is saturated, 
and when, therefore, an ascending current will always keep forming cloud, 
the effect of vapour of water, however near saturation, will be scarcely 
sensible on the cooling effect of expansion.” f 
§ 17. But, considering our terrestrial atmosphere as a whole, and the 
complicated circumstances of winds, and rain, and snow, and its heatings 
by radiations from the sun, and its coolings by radiation into starlit space, 
and its heatings and coolings by radiations to land and sea in different 
latitudes, we may feel sure that Joule’s suggestion shows a cause con- 
tributing importantly to the general average temperature-gradient being 
less than it would be in dry air in convective equilibrium. 
§ 18. For the solar atmosphere, we have approximately, g = 28 (28 times 
middle latitude gravity at the earth’s surface). By way of example, we 
may take S and k the same as for the terrestrial atmosphere, as we have 
not sufficient knowledge from spectrum analysis to allow us to guess other 
probable values of S and k for the mixture of gases constituting the upper 
parts of the sun’s atmosphere, than those we know for the mixture of 
Oxygen, Nitrogen, Argon, and Carbonic Acid, which in the main constitutes 
our terrestrial atmosphere. Thus in the upper atmosphere of the suiq 
* Quoted from the Manchester paper above referred to, Math, and Phys. Papers,. 
vol. iii. p. 260. 
t Ibid. 
