380 Original Articles. [July> 



effect of the vapour in arresting the outflow of heat may be greater. 

 It is possible, however, that his atmosphere may be peculiar in quality, 

 and may contain elements more capable of resisting the waste of heat 

 than exist on ours. And these may not be beyond the ken of spectral 

 analysis. So that without any unlikely suppositions we are provided 

 with a fair explanation of the comparatively mild climate of Mars, 

 and may expect from further research sure measures of the extent of 

 his atmosphere, and certain knowledge of its constitution and action 

 on radiant heat. If we could be quite confident of the exactness of 

 some observations of Cassini and Koemer, on the dimness and almost 

 extinction of stars before and after occultation by Mars, we should 

 have no doubt of his being surrounded by an extensive and partly 

 cloudy atmosphere ; but Sir J. South found no such obscuration, and 

 concluded that either some physical change has occurred in the atmo- 

 spere of that planet, or that the observations of Cassini were inaccurate. 

 Perhaps the safe conclusion may be that the atmosphere exists, but 

 is not always loaded with clouds, and this is in harmony with all that 

 appears on other grounds to be likely.* 



The greater our knowledge of the true constitution of Mars, the 

 better chance have we of ascertaining the physical peculiarities of 

 the other planets. We must not expect discoveries on them to 

 proceed at so great a rate as on Mars. Mercury is too near the sun 

 to be well seen. Uranus and Neptune too far from the earth. Of 

 these planets, even the rotation period is not ascertained. Schroter, 

 indeed, not only speaks of spots on Mercury, but gives measures of 

 mountains rising above his surface, one more than one mile, and 

 another nearly eleven miles, and assigns a rotation period ^of 

 24h. 5m. 28s. But in this, and in other statements, he is quite un- 

 supported. 



No great amount of definite knowledge is acquired regarding the 

 surface of Venus or Saturn, though for these the rotation period has 

 been assigned by means of some shadowy parts, whose recurrence 

 has been observed, but not by modern instruments. 



To Huygens, looking through a telescope 45 or 60 feet long, Venus 

 appeared " all over equally lucid, without so much as one spot in 

 her ; " and after satisfying himself that no distinction of land and 

 water could be traced in her, he asks, " Is not all that light we see 

 reflected from an atmosphere surrounding Venus, which, being thicker 

 and more solid than that in Mars or Jupiter, hinders our seeing any- 

 thing of the globe itself, and is at the same time capable of sending 

 back the rays that it receives from the sun.""]" 



Yet in 1666, Cassini saw a spot on the edge like a bright 

 prominence on the moon. In 1700, De la Hire discovered mountains 

 in Venus larger than those of the moon ; and Schroter not only saw 

 mountains, but measured them.J Tour of these were found to be 

 elevated from 10 to 22 miles. Modern observers have not confirmed 

 these statements ; yet, during the present year, we have seen such 



* ' Proceedings of the Royal Society,' 1831, 1832. 

 t ' Cosmo theoros,' first published 1684. 

 % ' Phil. Trans.' 1792. 



