AND THE OTHER PLANETS OF THE SOLAR SYSTEM. 667 



that planet compared with that of the Earth (such as the appearances of the planet would 

 seem to render probable) might render a positive value of t of considerable magnitude. The 

 quantities t' and t" must manifestly (unless especially affected by any local causes) be 

 positive, and may possibly amount to several degrees ; but the most important term in each 

 of the above expressions is the last. Thus suppose the height denoted by a to be 25000 feet, 

 a little more only than Gay Lussac and Mr Welsh ascended in their balloons ; the consequent 

 reduction of the mean superficial temperature for any latitude would amount to about 46° (C), 

 so that the mean temperature of the equinoctial regions of Venus would be actually reduced to 

 that of the temperate regions of the Earth, while that of her polar regions would equal 

 about 49° (C) reduced by a quantity depending on the values of t and t'. If we suppose 

 this reduced value to be about 40° (C), it will exceed the mean temperature of our own 

 tropical region by 12° or 13° (C). 



It must be recollected that the temperatures here spoken of are mean annual temperatures, 

 such as would be indicated by a thermometer shaded from the direct rays of the Sun. An 

 inhabitant of Venus would also experience during the day the heat of sunshine, like ourselves, 

 but of about double the intensity, supposing her atmosphere to be as diathermanous to the 

 Sun's rays as that of the Earth. But this supposition seems, as above intimated, to be 

 contradicted by the general appearance of this planet. The Moon, without any sensible 

 atmosphere, exhibits to us her mountains in minute detail, and Mars indicates the out- 

 lines of apparent continents ; but Venus, when examined even through the best telescopes, 

 exhibits very little more than a dazzling and uniform brightness, which has generally been 

 attributed to the reflexion from an atmosphere of small comparative transparency, and there- 

 fore hiding the real surface of the planet. This does not imply a more extended atmosphere 

 than that of the Earth, but rather one containing that larger amount of aqueous vapour which 

 would necessarily result in an atmosphere like our own from increased heat and evaporation. 

 It would seem probable therefore that the glare of sunshine on this planet may be so far 

 modified as not to be materially greater than on the Earth. 



We have still to consider the periodical inequalities of temperature, which, with the above 



assumed obliquity, become much more important than on our own globe. The most important 



will be the annual inequality, the maximum value of which will 



b it 

 = ± h — - sin 75° sin n, (m = lat.). 



Taking the value of — = ,731, the same as for the Earth, and h = 70° (C), this becomes 



± 77°,5 sin n. 

 The maximum value of the semi-annual inequality 



Taking — the same as for the Earth, its value is ,66 ; and taking the value above given of 



Q„ we have the greatest value of the semi-annual inequality at the equator of Venus 



= ± 16°,4 (C). 



