The next point that ought to engage our attention is the atmosphere, without which no life is possible. The method we use to determine whether a planet has an atmosphere is a very simple one: we have only to observe it pass in front of a fixed star; then, if there is no air round it, the light from the star will be extinguished instantaneously, as it is in the case of the moon; whereas, if it has an atmosphere, the light will gradually die away; because, instead of being cut off suddenly by an opaque body, it will be slowly diminished by the increasing thickness of the air that it is viewed through, and will very likely have entirely disappeared before the actual body of the planet is interposed. By applying this observation to Mars, it has been determined that it has an atmosphere, the exact thickness of which, however, we are unable to measure. It seems fair to assume that the amount of air which surrounds it is about the same proportion to the total mass of the planet as in the case of the earth. Without entering into calculations, we may state that if this is true, the pressure of the air at the surface of Mars would be about equal to five inches of mercury, or about one-sixth of the normal atmospheric pressure on the earth. Now, given an atmosphere and a large extent of sea, we should naturally expect that clouds would form a prominent feature on the Martial surface; and observation has proved this to be the case. On several occasions, some of the features of the planet have been observed to be obscured by a sort of white film, which it is only fair to assume was a cloud. These clouds appear more markedly at the edge of the disc, or at those points where it would be morning or evening, and we may therefore assume that, similar to the earth, Mars is liable to mists or clouds forming at dawn and in the evening. It has been suggested that these white films are due not to clouds in the air, but to a deposition of snow on the surface, which disappears when the sun rises. There seems to be no particular reason for adopting this theory; it does not explain the observed phenomena better, nor does it seem more likely to be true. The air on Mars being very much less dense than on the earth, it is presumable that the winds would move with much greater velocity; and for this reason, it has been thought that trees could not grow to any considerable height. We must, however, bear in mind that though the velocity would be high, the actual force of the wind would probably not be very great, on account of its excessive tenuity.