1873J The Planet Mars in 1873. 193 



surface than there is above each square mile of the earth's 

 surface. For the density of the air at the sea-level is pro- 

 portional to the weight of the air above each unit of surface. 

 For this weight, in the case of Mars, to be the same as in 

 the case of the earth, the quantity of air above each unit of 

 surface must be greater in the proportion of 1000 to 387, 

 that being the proportion in which terrestrial gravity exceeds 

 gravity at the surface of Mars. Taking 18 to 7 as suf- 

 ficiently near, we have the following consequences if we 

 assume that at the surface of Mars the atmospheric pressure 

 is the same as on the earth. We have in the first place a 

 coating of air, which is greater in quantity, -square mile for 

 square mile, than on the earth in the proportion of 18 to 7. 

 But it must also be correspondingly greater in depth, for 

 we know that on the earth the pressure is halved at a 

 height of 3J miles, in other words that half the atmosphere 

 lies below this height. At seven miles ,the pressure is 

 reduced to one-fourth — that is, three-fourths of the air lie 

 below this level : and so on. Now, in the case of Mars, 

 the reduction proceeds in the same way, but at different 

 heights. We must increase 3J in the proportion of 18 to 7 

 to obtain the height above the mean surface of Mars, at 

 which the atmospheric pressure is reduced one-half. This 

 gives nine miles as the elevation required. At a height of 

 18 miles, the pressure is reduced to one-fourth ; and so on. 

 Now on our assumption as to the actual quantity above 

 each square mile of the surface of Mars, the region above 

 the mean level of the planet to a height of 18 miles is 

 occupied by air, having a mean density as great as that of 

 the air below the height of seven miles from the terrestrial 

 sea-level. Moreover, if we assume a height of 35 miles 

 only as that to which the optically effective atmosphere of 

 the earth extends, we get for the corresponding height in 

 the case of Mars no less than 90 miles. Now, remember- 

 ing that the diameter of Mars is but about 4400 miles, it 

 seems clear that an atmosphere so deep as this should be 

 telescopically recognisable. 



But this is not all : if Mars had an atmosphere no denser 

 at the sea-level than the terrestrial atmosphere, he would 

 not have the same climate as the earth ; for as we have seen 

 the solar light and heat at Mars are reduced in the pro- 

 portion of about 4 to 9 as compared with the solar light and 

 heat at the earth. A very much denser, and therefore a very 

 much deeper, atmosphere than that deduced above would be 

 required to produce a Martial climate resembling our own ; 

 and even then, it may be questioned whether with his 



vol. in. (n.s.) 2 c 



