Otayo Institute. 435 
that the conditions under which life is now maintained on the earth are 
derived from the additions which the solar radiation constantly makes to 
the intrinsic heat of our planet. 
It becomes then a point of the highest interest to ascertain the actual 
temperature of this great ruler of our system, what its present rate of 
cooling would be if that temperature were not continually restored, and 
what means of maintaining that temperature appear to exist. It is with 
the first of these that we have to do, but it may not be without interest to 
state here one of the most probable estimates that has been made as to the 
other two. Ericsson has calculated that, if the contraction which constant 
radiation of heat must necessarily effect in the sun, should proceed at the 
rate of one foot of his radius in 8 days and about 34} minutes (3-024 
days), the development of heat would equal the radiation from the solar 
surface, as he determines it. Contraction at this rate would in about 
2,000,000 years reduce the sun’s diameter by one-tenth. The thermal 
energy at the sun’s surface would, during the whole period, be maintained 
constant at its present figure, but the diminished size of the solar dise 
would result in a diminution of the temperature communicated to the 
earth, at its present distance, by an average of nearly 13° Fah. Although 
this would involve a notable change in the conditions of life on the earth, 
and would probably be sufficient to depopulate its arctic regions, the change 
would not reduce the average temperature of tropical regions to that which 
at present prevails in Dunedin. This is, of course, all pure speculation, 
because the data assumed cannot be verified. My object to-night is indeed 
to show that they are very unreliable. We may, nevertheless, rest content 
in the conviction that the secular cooling and contraction of the sun, if it 
should actually be proceeding in the manner which Ericsson assumes, will 
haye no effect on the conditions of life upon the earth within a period 
utterly beyond our powers of conception; for, though we can express 
millions of years in numbers, the idea which we form in our minds of such 
a lapse of time is really of the vaguest description. At any rate, there 
appears to be every reason to believe that before the earth becomes too 
cold to be inhabited by beings like ourselves, there will be ample time for 
‘‘gocial evolution ” to carry our race to that future of perfect development 
towards which we are taught to believe that it is inevitably tending. 
The subject we have in hand excited, not long ago, considerable attention 
amongst scientific men, on account of the publication by Father Secchi, in 
his work, ‘Le Soleil” (Paris, 1870), of his estimate of the solar tempera- 
ture as not less than 10,000,000° Cent, (say 18,000,000° Fah.) The learned 
Director of the Roman Observatory, in the calculations by which he arrived 
at this enormous temperature, rejected the received law of radiation estab- 
