696 Lord Kelvin : The Problem of 
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 consti- 
tuting 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 sun, if in 
purely convective equilibrium, and undisturbed by radiations 
and other complications, the temperature would increase 
at the rate of 280 degrees Centigrade per kilometre down- 
wards, and, looking forward to § 27 below, we see that the 
increase of temperature would start from absolute zero at 
the boundary, where density, pressure, and temperature, are 
all zero. It would require very robust faith in the suggestion 
of convective equilibrium for the gaseous atmosphere of the 
sun to believe in + 7° C. being the actual temperature of 
the sun's atmosphere at one kilometre below the boundary. 
I am afraid I cannot quite profess that faith. It seems to 
me that the enormous radiation from below would, if the 
upward and downward currents were moderately tranquil, 
overheat the air in the uppermost kilometre of the sun's 
atmosphere to far above the temperatures ranging from 
— 273° Centigrade to +7° Centigrade, calculated as above 
from the adiabatic convective theory. 
§ 19. Keeping, however, for the present by way of example, 
to the calculated results of this theory, with the data for S 
and k chosen in § 15, we find at ten and at fifty kilometres 
below the boundary, the temperatures, reckoned in Centi- 
grade degrees above absolute zero, would be respectively 
2800 and 14000. Calling these temperatures t' and t, and 
the densities at the same places p' and p, we find by (14) 
MW— • • • • <">• 
Suppose for example p' to be '001 (1/1000 of the density of 
water), we shall have p — ' 05 6. This last is nearly but not 
quite too great a density for approximate fulfilment of the 
gaseous laws for the same gaseous mixture as our air. Thus, 
if not too much disturbed by radiation of heat from below, 
the uppermost fifty kilometres of the sun's atmosphere might 
be quite approximately in gaseous convective equilibrium ; 
with density and temperature augmenting from zero at the 
boundary, to density '056, and temperature 14000 Centi- 
grade degrees above absolute zero, at the fifty kilometres 
