120 Proceedings of Royal Society of Edinburgh. [mar. 7, 
to keep up about the same radiation effect, though its temperature 
may have fallen greatly. 
It is evident that much of this is purely theoretical and formed 
on rather a weak basis; we have so few measurements to go by. 
Though it is a law that compound bodies radiate more than simple 
ones, yet we do not know enough of the constitution of the sun to 
say how much the increased radiating power, due to increased com- 
plexity, would compensate for the fall of temperature. I have said 
it might actually more than counteract the fall of temperature and 
cause an increased radiation effect. It may, however, only balance 
it, or it may even only reduce the rate of decrease of radiation, 
and make it only a little less than proportional to the fall in tem- 
perature. 
The whole of these remarks are almost pure speculation. The 
principal cause for their being written is to point out that the radiat- 
ing power of the sun may have varied in quantity and quality from 
age to age ; that its amount may not be directly proportional to its 
absolute temperature ; and further, that it is extremely doubtful 
whether we can apply to solar matter the radiation measurements 
which have been obtained of earth matter, so that any estimates we 
may make of the temperature of the sun from measurements of 
solar radiation must be received with considerable hesitation. 
Added 9 tli July 1887. 
Sir William Thomson has shown that the sun has within itself 
an enormous store of energy due to its high temperature. This 
energy is altogether apart from what we on the earth are accustomed 
to consider as the energy stored up in a hot body. He has 
shown that the shrinkage or falling in of the great mass of the 
sun due to cooling is capable of developing a very great amount of 
energy. His calculations show, that if the shrinkage was so great 
that the surface of the sun was to descend at the rate of 35 metres 
per year, or 70 kilometres per 2000 years, there would be developed 
about the same amount of energy that is radiated by the sun accord- 
ing to the measurements taken by Pouillet. 
Forbes has shown Pouillet’s measurements to be much too small, 
and that the amount radiated by the sun is 1*6 greater than 
