232 
POPULAR SCIENCE REVIEW. 
saw a similar phenomenon in 1828, having noticed a dark cloudy 
spot to have changed its position, in an interval of four days, 
from the side to the top of the planet (see diagram). One very 
persevering observer, however (Maedler), thinks that many very 
natural mistakes may occur on this point, and considers that if the 
spots are not narrowly scrutinized each night, one of them may 
readily be mistaken for a cloud. This observer has, however, 
noticed that the spots on Mars are of varying colours at dif- 
ferent times ; some of them are black, others yellowish-red, 
and others again of a greenish tint. On the Earth, however, 
the same change of tint would be apparent ; the great forests 
would be dai'k in comparison with the bright sandy deserts ; 
the Black, White, and Red seas might appear of different tints ; 
yellow fogs and the “ brickdusters ” of Australia would cause 
a proportionate change in the apparent colour of the countries 
which they enveloped, and over which they hung. This is 
perhaps a more natural explanation of the colour of Mars than 
that which accounts for its peculiar appearance by the tints 
assumed by the vegetation on the planet. Without, however, 
having recourse to accidental fogs and clouds, it has been 
supposed that the dense atmosphere of the planet absorbs all 
the violet rays of the solar light (which has to pass twice 
through its atmosphere before reaching the Earth), and that 
only the red rays — those of the rising and setting Sun when 
passing through dense mist or cloud — make their way to our 
planet, the others being either reflected or absorbed. 
Let us now turn our attention to the snowy zones about the 
north and south poles of the planet. By direct measurement 
with an instrument for estimating- light, it has been found that 
those white spots have twice the intensity of the dun-coloured 
portions of the planet. In the earliest years of telescopic 
discovery they were duly noticed, and their appearance and dis- 
appearance, and the various changes which they underwent, 
were curiously scrutinized for a considerable length of tune, 
before, however, any one surmised that the chang-es were caused 
by atmospheric variations. By his observations between 1779 
and 1784, the elder ILerschel placed this matter beyond all 
doubt. In the latter year he detected all the changes which 
have since been observed and confirmed in this planet ; and 
although Maraldi had seen the changes, and prophesied the 
extinction of the north snow-spot as far back as 1719, yet to 
Herschel is due the credit of them complete examination. 
He was the first to show that the snow was not exactly placed 
at the poles of the planet. This is very apparent from our 3rd, 
4th, and 5th figures, which have been very carefully drawn by 
Secchi ; the first in 1856, and the two latter in 1858, with fine 
optical and atmospheric circumstances in his favour, and which. 
