Slipher’s discovery was considered so valuable that it must be employed to the limit. The well-known physicist Very was therefore called in consultation; he made careful measurements of the intensity of the “A”-lines on the various plates and calculated that the atmosphere of Mars contained 1.75 times as much water vapour as that of the Earth at the point of observation. If we desire to determine the proportion of water vapour in the air at the surface of Mars from this statement we may figure in the following manner. The amount of water vapour in a vertical column of air one square meter (1.2 sq. yds.) in section is, according to Hann, 2500 times the amount in a cubic meter (1.3 cu. yds.) at the surface of the earth. At the time of the observation, the latter amount was 2.29 grammes (35.4 grains); on each square meter (1.2 sq. yds.) of the ground rested therefore 5725 grammes (12 lbs. 11 oz.) of water vapour. That the quantity of water is not larger, although the depth of the atmosphere far exceeds 2500 m. (1.5 miles), is due to the fact that the temperature rapidly decreases with distance from the ground. On Mars, the temperature ought not to fall so quickly with change in height because the intensity of gravity there is 2.68 times smaller than on the Earth. The temperature drops there 2.68 times slower with ascent in the atmosphere, and a column of air on Mars one square meter (1.2 sq. yds.) in section should therefore contain 6680 times as much water vapour as a cubic meter (1.3 cu. yds.) at its surface. As Mars did not stand in zenith, the distance traversed by the light-ray in the atmosphere was greater--in fact 1.43 times greater than if such had been the case. A column of air in the direction of the light-ray, one square meter (1.2 sq. yds.) in section contained therefore 8175 grammes (18 lbs. 3 oz.) water vapour. In the atmosphere of Mars which the light passed in a vertical direction there was, if we are to believe Very, 1.75 times as much, or 14,300 grammes (31 lbs. 8 oz.) and in a cubic meter (1.3 cu. yds.) at the surface of the planet, consequently 6680 times less, or 2.14 grammes (33.1 grains). The corresponding dew-point is then, according to this determination, -10.3° C. (+13.5° F.). It is agreed upon that a desert climate prevails on Mars. It might at the time of the observation conform to the extremely dry climate at Salt Lake City in the height of summer when the humidity there is only 31 per cent. of saturation. Under such conditions saturated air at noon in the equatorial belt on Mars should contain 7 grammes (108 grains) per cubic meter (1.3 cu. yds.) corresponding to a temperature of 5.3° C. (41.5° F.).