188 The Planet Mars in 1873. [April, 



opposition, the permanence and nearly equable distinctness 

 of the principal features, under similar circumstances, was 

 surprising. On no occasion could I satisfy myself that any 

 part was decidedly less distinct than might be expected 

 from the appearance of the other features then visible. 

 The very white spots noticed on a few occasions, which 

 certainly gave the impression of masses of snow or the 

 reflection from the upper surface of masses of cloud, formed 

 the only decided exception, unless we include the somewhat 

 remarkable fact that the short and rather thick dark line 

 plainly seen near the North Pole on November 14th was 

 invisible on the 12th, when the narrow strait extending from 

 that part of the northern hemisphere towards the south 

 and other objects in the same vicinity were well seen. On 

 November 10th, also, the northern extremity of that narrow 

 strait was invisible, though it might have been expected to 

 be quite as well seen as on the 12th, and even better than 

 on January 22nd. These exceptions to the prevalent clear- 

 ness of the Martial atmosphere, both relate to regions in 

 high Martial latitudes, and therefore literally tend to ' prove 

 the rule.' " 



We come next to the very natural and effective expla- 

 nation of the Martial snow-caps, in Mr. Williams's theory. 

 We have seen how, under the supposed circumstances, 

 there would be a deposition of hoar-frost continually taking 

 place all round the disc of Mars. Now, " the rotation of 

 the planet will produce," as Mr. Williams points out, " a 

 considerable difference in the results of this deposition. All 

 that falls on the east and west sides of the planet will be 

 thawed and evaporated by the next day's sunshine,* so that 

 the maximum accumulation in these directions can be but 

 one night's deposition ; but on the north and the south there 

 will be continual accumulation, which will only be thawed 

 up to a certain latitude by the annual summer presentation 

 of either hemisphere to the sun." The distance between 

 the mean limits of the north and south patches of accu- 

 mulated hoar-frost may be taken as an approximate measure 

 of the diameter of the circle over which the sun's rays are 

 capable of raising the day-time temperature above the 

 freezing-point (or rather perhaps, of melting quite through 

 the deposited layer of light snow)." Here Mr. Williams 

 notes a consideration which suggests an interesting point 

 for observation. He remarks that the boundary of the 



* The part on the west is actually coming into sunshine, so that " the day's 

 sunshine " would be a more correct expression than " the next day's sun- 

 shine," as respects this part of the planet. 



