“ On the Temperature Equilibrium of an Enclosure con- 
taining a Body in Visible Motion,” by Balfour Stewart,' 
LL.D., F.R.S. 
It has been established that in an enclosure containing 
bodies which are all at the same temperature, and at rest, 
the same amount of heat enters any surface forming part of 
the walls of the enclosure as leaves it in the same time, so 
that the body, of which this is the surface, neither gains nor 
loses heat. It is also known that if we take Dot the outer 
surface of such a body, but any plane passing through its 
substance; say for instance one parallel to its outer surface, 
then, as much heat passes across this plane going into the 
bod}'', as passes across it going out of the body in the oppo- 
site direction; and further, this equilibrium of heat is known 
to hold seperately for every one of the individual rays of 
which the whole heterogeneous radiation is composed. 
The effect of the motion of a body in altering the wave- 
length of the radiated light is also well known. In 
consequence of this, if a cosmical mass, such as a star or 
nebula should be formed of incandescent hydrogen, and be 
at the same time rapidly approaching the earth, the light 
which strikes the earth will not be the double line D, but a 
line more refrangible than it, and therefore this light will 
be able to pass through a mass of ignited sodium vapour at 
the earth’s surface without suffering absorption, while, how- 
ever, the light emanating from the sodium vapour will still 
be the double line D. 
In such a case even if the star and the terrestrial sodium 
vapour should both be of the same temperature, yet the light 
radiated by the latter will not be the same in quality as that 
absorbed. This instance would appear to show that the 
equilibrium which holds in an enclosure of uniform tem- 
perature when all the substances are at rest does not hold 
when some of these are in visible motion, and that if in that 
enclosure there be a body moving towards or from the sur- 
