The air supply proved also to be quite satisfactory. The _Reiset_ and _Regnault_ apparatus for producing oxygen contained a supply of chlorate of potash sufficient for two months. As the productive material had to be maintained at a temperature of between 7 and 8 hundred degrees Fahr., a steady consumption of gas was required; but here too the supply far exceeded the demand. The whole arrangement worked charmingly, requiring only an odd glance now and then. The high temperature changing the chlorate into a chloride, the oxygen was disengaged gradually but abundantly, every eighteen pounds of chlorate of potash, furnishing the seven pounds of oxygen necessary for the daily consumption of the inmates of the Projectile. Still--as the reader need hardly be reminded--it was not sufficient to renew the exhausted oxygen; the complete purification of the air required the absorption of the carbonic acid, exhaled from the lungs. For nearly 12 hours the atmosphere had been gradually becoming more and more charged with this deleterious gas, produced from the combustion of the blood by the inspired oxygen. The Captain soon saw this, by noticing with what difficulty Diana was panting. She even appeared to be smothering, for the carbonic acid--as in the famous _Grotto del Cane_ on the banks of Lake Agnano, near Naples--was collecting like water on the floor of the Projectile, on account of its great specific gravity. It already threatened the poor dog's life, though not yet endangering that of her masters. The Captain, seeing this state of things, hastily laid on the floor one or two cups containing caustic potash and water, and stirred the mixture gently: this substance, having a powerful affinity for carbonic acid, greedily absorbed it, and after a few moments the air was completely purified. The others had begun by this time to check off the state of the instruments. The thermometer and the barometer were all right, except one self-recorder of which the glass had got broken. An excellent aneroid barometer, taken safe and sound out of its wadded box, was carefully hung on a hook in the wall. It marked not only the pressure of the air in the Projectile, but also the quantity of the watery vapor that it contained. The needle, oscillating a little beyond thirty, pointed pretty steadily at "_Fair_." The mariner's compasses were also found to be quite free from injury. It is, of course, hardly necessary to say that the needles pointed in no particular direction, the magnetic pole of the Earth being unable at such a distance to exercise any appreciable influence on them. But when brought to the Moon, it was expected that these compasses, once more subjected to the influence of the current, would attest certain phenomena. In any case, it would be interesting to verify if the Earth and her satellite were similarly affected by the magnetic forces.