( 680 ) 
so that 
4 1 
dv, — —— fn... 
3 Bane. 
The corresponding energy per unit of space is 
1 1 
nd + — $= — Hy. . @ 
and, according to PoyNTING’s theorem, there is a flow of energy 
along OX 
¢ 
c2 d, aE = 7e He, 
this quantity being the amount of energy per unit of time and unit 
of area, which traverses an element of surface, perpendicular to OX, 
and not moving with the earth. 
In what follows we have only to attend to the mean values of 
the energy and its flow, taken for a full period or for a lapse 
of time, embracing a large number of periods. The mean value 
of (2) is 
De ORE Ae 
Sic? e J rt! 
and for that of the energy-current we may write 
ee 
Since v may be negative as well as positive, the above formulae 
apply not only to the vibrations, sent out in the direction of the 
earth’s motion, but equally to those which go forth in opposite direction. 
De, a ae 
The factor 1 + — in the expressions for d, and 9. is different in 
c 
the two cases; it would however be rash, to conclude from this, 
without closer examination, that the difference will make itself felt 
in measurements on the heating of a body exposed to the rays. 
Let there be, in any point of the positive axis of 2, and placed 
perpendicular to it, a disk of infinitely small area @, and composed 
of a perfectly black material, so that it reflects no part of the incident 
radiation. This disk will be supposed to be fixed to the earth, and 
we shall deduce the amount of heating from the law of conservation 
of energy, taking into account that the rays exert on the disk a 
certain normal pressure, the amount of which per unit area is given 
precisely by U). 
Imagine a right cylinder C, having @ for its base and turned 
towards the source of heat, and suppose the face of it, that is opposite 
1) See e.g. my «Versuch u. s. w.”, §§ 16 and 17. 
