1868. ] Astronomy. 217 
Dawes and Browning at the other. Hooke took two closely according 
views separated by an interval of 10 minutes; Dawes’ accuracy of 
delineation is beyond question ; and though Browning has had less 
practice in observations of this sort, yet he has already acquired a 
high reputation for accuracy and care, and further his drawing gives 
results closely according with those deduced from Dawes’ views. 
Hence we can scarcely doubt that the mean of the above rotation-periods, 
or 24h. 37m. 22°735s. is very near the true rotation-period of Mars. 
We notice, however, that an error of 1° im any of the corrections 
corresponds to an error of about #6 x stoths (or very nearly zioth) 
of a second, or ‘0036s. ; so that Mr. Proctor’s result cannot be held 
to be trustworthy beyond the second place of decimals,—perhaps 
even not beyond the first. We may assume that Mars’ rotation- 
period lies certainly between 24h. 37m. 22°75s. and 24h. 37m. 
22-71s., and a corresponding error therefore exists in Kaiser’s 
estimate—24h. 37m. 22°6s., and Madler’s—24h. 37m. 23°7s. 
Mr. Harrison supplies an interesting paper on the moon’s 
insolation. Assuming that the moon’s substance has a capacity 
for heat, the mean maximum state of insolation of any hemisphere 
would evidently be attained when the largest surface has been 
continuously exposed to the sun’s heat for the longest duration 
of time. Accordingly, the visible hemisphere would be heated to 
its greatest possible extent during the third, or last quarter, when 
the half-moon then illuminated has been subjected to the solar 
radiation for a mean period of 265°5 hours, and the remaining 
half, now in shade, has very recently received the sun’s rays for 
a period of equal duration. Now, the heat thus assumed to be 
acquired by the moon, and radiated to the earth, is dark heat; 
and it has been shown by Professor Tyndall that the aqueous 
vapour in our atmosphere has a power of absorbing dark heat. 
Accordingly, instead of heating the earth, this heat would be em- 
ployed in heating the air above the clouds, and causing increased 
evaporation from their surface. Clouds would thus be raised to a 
higher elevation, and, under favourable circumstances, would even 
be dispersed. ‘Thus, there would be a diminished check upon the 
radiation of terrestrial heat, and a sensible fall in the temperature 
of the air near the ground would necessarily follow ; and results 
of a precisely opposite character would occur at the period of 
minimum heat in the moon’s visible hemisphere. The daily mean 
temperatures at the Oxford, Berlin, and Greenwich Observatories, 
when arranged in tables according to the age of the moon, show 
that the temperature of the air near the ground is sensibly affected. 
The maximum mean temperature occurs on the average on the 
sixth and seventh day of the lunation, and the minimum soon 
after fuil moon—results which confirm Mr. Harrison’s views in 
a remarkable manner. 
