tube, would give the means of regulating the air-passage at pleasure. The difficulty of this simple work, and the tremendous outward pressure of the air, showed that the external atmosphere was very thin indeed. This I had anticipated. Gravity on the surface of Mars is less than half what it is on Earth; the total mass of the planet is as two to fifteen. It was consequently to be expected that the extent of the Martial atmosphere, and its density even at the sea-level, would be far less than on the heavier planet. Rigging the air-pump securely round the aperture, exhausting its chamber, and permitting the Martial air to fill it, I was glad to find a pressure equal to that which prevails at a height of 16,000 feet on Earth. Chemical tests showed the presence of oxygen in somewhat greater proportion than in the purest air of terrestrial mountains. It would sustain life, therefore, and without serious injury, if the change from a dense to a light atmosphere were not too suddenly made. I determined then gradually to diminish the density of the internal atmosphere to something not very much greater than that outside. For this purpose I unrigged the air-pump apparatus, and almost, but not quite, closed the valve, leaving an aperture about the twentieth part of an inch in diameter. The silence was instantly broken by a whistle the shrillest and loudest I had ever heard; the dense compressed atmosphere of the Astronaut rushing out with a force which actually created a draught through the whole vessel, to the great discomfiture of the birds, which roughed their feathers and fluttered about in dismay. The pressure gauge fell with astonishing rapidity, despite the minuteness of the aperture; and in a few minutes indicated about 24 baro-
