D8 Transactions. — Miscellaneous, 



The last-mentioiied kind is by far the most favoured. The eminent 

 astronomer, Secchi, expresses an opinion that only a very small portion of 

 the heat of the sun can be derived from this source ; but the reasoning on 

 this point does not seem conclusive, and is not, so far as I am aware, 

 acquiesced in by other physicists. 



We see this process constantly occurring on our earth, small bodies 

 constantly plunging into our atmosphere, becoming incandescent by the con- 

 version of their motion, and showing then* course by a more or less vivid 

 streak of light. Following the opinion of more than one astronomer, I 

 assume that the process indicated is not confined to the earth, but is common 

 to the latter with all other heavenly bodies. 



We have considerable reason for supposing that meteoric matter is dis- 

 tributed throughout space, being perhaps sparse in some parts, and mode- 

 rately closely aggregated in other parts, more condensed in the neighbour- 

 hood of large centres of attraction than at a distance from such centres. 

 Assuming this, it will follow that every body in its progress through sj)ace 

 will be continually bombarded with meteoric particles. 



The quantity each body will receive at any time will be proportioned — 

 firstly to the richness of the meteoric field ; secondly, to the transverse 

 section of the body, and thirdly, to the attractive energy of the body, in 

 other words to the mass. 



The amount of heat generated by the impact of a given mass will be 

 proportional to the mass of the attracting body. 



It will thus be seen that the impact of say one pound on say Jupiter, 

 will generate more than three hundred times more heat in that planet than 

 a similar impact will generate on the eaith, the masses of those planets 

 being as more than 300 to 1. 



It is true, that as there is more than three hundi'cd times as much matter 

 in Jupiter to be heated, the absolute rise in temperature of the whole mass 

 will be the same in both instances ; but a larger proportion of the heat 

 generated will escape by radiation from the smaller body in a given time 

 than from the larger — this is readily seen. 



Taking approximate figures, the diameter of the earth being 1, the sur- 

 face 1, and the mass 1 ; the diameter of Jupiter is 11, the surface 121, 

 and the mass 300 : — the amount of heat generated by the impact of a given 

 mass will be on the earth 1, on Jupiter 300. 



The amoimt of heat radiated away is proportional to the surfaces fi'om 

 which radiation takes place. In the case of the earth 1, in the case of Jupi- 

 ter 121. So that while Jupiter receives three hundred times the amount 

 of heat energy received by the earth, it loses in a given time one hundred 

 and twenty-one times, leaving a residue of one hundred and seventy-nine 



