fore, it might appear that there was an increase in the calculated diameter, or it might seem that there was a decrease from it, and either would be equally likely to happen. if, however, many measures were made, and just in proportion as they were many, those decreasing the diameter would offset those increasing it, and the mean of all would show no trace of either. In the mean the minus quantity would wipe out the plus. Indeed, owing to the fact that both the Sun and the Earth are not infinitely far off from Mars, and in consequence that all the lines to them are not strictly parallel to one another, the decreasing effect would actually slightly exceed the increasing effect, but this would be too small to be perceptible.
The same argument that applies to mountains applies to clouds, or to any opaque substance. Sporadic increase might be due to them; but for the increase to be systematic, it is necessary that the substance seen should also be seen through. It must be in part transparent. The measures, therefore, not only disclose the presence of an atmosphere, but do so directly.
Having thus seen first with the brain and then with the eye, and both in the simplest possible manner, that a Martian atmosphere exists, we will go on to consider what it may be like.