672 Mr W. HOPKINS, ON THE EXTERNAL TEMPERATURE OF THE EARTH, &c. 



Hence, then, it appears that the superficial temperature of the Moon's mass, in the imme- 

 diate vicinity of her equator, must have a mean value exceeding that of surrounding stellar 

 space, by an amount which may probably be estimated at nearly 40° (C), and will be subject 

 to an enormous monthly inequality, which may possibly amount to upwards of 30° (C) in 

 excess or defect, and therefore producing in the extreme monthly temperatures a difference of 

 some 60° (C). It must, however, be recollected that this conclusion involves the hypothesis 

 that the values of c, k, and p, the specific heat, conductivity and radiating power, are the same 

 for the Moon as for the Earth. At the poles the mean temperature can little exceed that of 

 stellar space, and the inequalities must be very small. 



The Moon's external temperature, or that which would be indicated by a thermometer 

 placed at any point not remote from her surface and external to it, must depend, in the 

 absence of a sensible atmosphere in contact with the bulb of the instrument, entirely on 

 radiation from the Moon herself, the Sun, and all other bodies in stellar space. If it received 

 heat from the stellar bodies alone, it would indicate the temperature above denoted by £ ; and 

 if it were then exposed also to the radiation from the Moon, but sheltered from that of the 

 Sun, the instrument would rise till the heat received by the bulb in consequence of inter- 

 radiation between the bulb and the Moon should equal that emitted in consequence of the 

 inter-radiation between the bulb and the stellar bodies. The indicated temperature, therefore, 

 would be less or greater than the superficial temperature of the lunar mass, according as the 

 latter should be higher or lower than that of stellar space; it would, probably, be much less, 

 when the superficial temperature should acquire its greatest monthly value. This defect, 

 however, of temperature might possibly be more than counterbalanced by the direct heat from 

 the Sun, when the instrument should be exposed to the solar as well as to the lunar and 

 stellar i - adiation. 



The monthly inequality of temperature above noticed is manifestly analogous to the daily 

 inequality in the terrestrial temperature. Its magnitude depends on the slow angular motion 

 of the Moon about her axis, and shews the remarkable degree in which a more or less rapid 

 rotation may affect the variations of temperature on any planet or satellite sufficiently near to 

 the central source of heat of our system. 



W. HOPKINS. 



