ON THE AGE OF THE SUN'S HEAT. 353 



estimating the possible amount of matter in the 

 zodiacal light ; and it was conjectured that it could 

 not be nearly enough to give a supply of 30,000 

 years' heat at the present rate. These anticipa- 

 tions have been to some extent fulfilled in Le 

 Verrier's great researches on the motion of the 

 planet Mercury, which have recently given 

 evidence of a sensible influence attributable to 

 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 ; 

 and, therefore, if the meteoric influx taking place 

 at present is enough to produce any appreciable 

 portion of the heat radiated away, it must be 

 supposed to come from matter circulating round 

 the sun, within very short distances of his surface. 

 The density of this meteoric cloud would have 

 to be supposed so great that comets could scarcely 

 have escaped as comets actually have escaped, 

 showing no discoverable effects of resistance, 

 after passing his surface within a distance equal 

 to 1/8 of his radius. All things considered, there 

 seems little probability in the hypothesis that 



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