OF GEOLOGICAL DYNAMICS. 129 



test whether or not there can be a sufficient amount 

 of matter circulating as meteors inside the orbit of 

 Mercury to provide sun-heat for a few hundred 

 years to come. Since that time Leverrier's fine 

 researches on the motions of the planet Mercury 

 give evidence of matter circulating as a great 

 number of small planets within his orbit round the 

 sun. But the amount of matter thus indicated is 

 very small, probably not enough for a few hundred 

 years' heat. It is therefore highly improbable that 

 the heat of the sun depends at all for its continua- 

 tion upon a continued meteoric supply. In the 

 present state of science what appears most prob- 

 able is Helmholtz's view, that the sun originally 

 acquired his heat in being built up out of smaller 

 masses falling together and generating heat by 

 their collision, but that at present he is simply 

 an incandescent mass cooling. In an article in 

 Macmillaris Magazine, March, 1862, " On the Age 

 of the Sun's Heat," he (Sir W. Thomson) had 

 shown that the sun may have been several million 

 future years giving out heat and light from the vast 

 initial supply generated in that manner ; but that, 

 VOL. II K 



