a Spherical Gi 
iseous Nebula. 
Temperature centigrade 
or* -273-7. 
Elevation from Earth's 
surface required to cool 
moist air by 1° C. 
civ 

-dt 

Metres 152 
5 
168 
10 
186 
15 
207 
20 
229 
25 
252 
30 
274 
35 
281 
695 
§ 16. From this we see that an ascending current o£ 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 contributing importantly to the general average tempe- 
rature-gradient being less than it would be in dry air in 
convective equilibrium. 
§ 18. For the solar atmosphere, we have approximately, 
(7 = 28 (28 times middle latitude gravity at the earth's surface). 
* Quoted from the Manchester paper above referred to, Math, and 
Phys. Papers, vol. iii. p. 260. 
t Ibid. 
3 A2 
