Lussac appears to have accomplished the exceedingly difficult task of counting the oscillations of his magnet with satisfaction to himself. At 10,000 feet twenty vibrations occupied 83 seconds, as compared with 84.33 seconds at the earth's surface. The variation of the compass remained unaltered, as also the behaviour of magnetised iron at all altitudes. Keeping his balloon under perfect control, and maintaining a uniform and steady ascent, he at the same time succeeded in compiling an accurate table of readings recording atmospheric pressure, temperature and humidity, and it is interesting to find that he was confronted with an apparent anomaly which will commonly present itself to the aeronaut observer. Up to 12,000 feet the temperature had decreased consistently from 82 degrees to 47 degrees, after which it increased 6 degrees in the next 2,000 feet. This by no means uncommon experience shall be presently discussed. The balloon was now steadily manoeuvred up to 18,636 feet, at which height freezing point was practically reached. Then with a further climb 20,000 feet is recorded, at which altitude the ardent philosopher could still attend to his magnetic observations, nor is his arduous and unassisted task abandoned here, but with marvellous pertinacity he yet struggled upwards till a height of no less than 23,000 feet is recorded, and the thermometer had sunk to 14 degrees F. Four miles and a quarter above the level of the sea, reached by a solitary aerial explorer, whose legitimate training lay apart from aeronautics, and whose main care was the observation of the philosophical instruments he carried! The achievement of this French savant makes a brilliant record in the early pages of our history.
It is not surprising that Lussac should own to having felt no inconsiderable personal discomfort before his venture was over.
In spite of warm clothing he suffered greatly from cold and benumbed fingers, not less also from laboured breathing and a quickened pulse; headache supervened, and his throat became parched and unable to swallow food. In spite of all, he conducted the descent with the utmost skill, climbing down quietly and gradually till he alighted with gentle ease at St.
Gourgen, near Rouen. It may be mentioned here that the analysis of the samples of air which he had brought down proved them to contain the normal proportion of oxygen, and to be essentially identical, as tested in the laboratory, with the free air secured at the surface of the earth.
The sudden and apparently unaccountable variation in temperature recorded by Lussac is a striking revelation to an aerial observer, and becomes yet more marked when more sensitive instruments are used than those which were taken up on the occasion just related. It will be recorded in a future chapter how more suitable instruments came in course of time to be devised. It is only necessary to point out at this stage that instruments which lack due sensibility will unavoidably read too high in ascents, and too low in descents where, according to the general law, the air is found to grow constantly colder with elevation above the earth's surface. It is strong evidence of considerable efficiency in the instruments, and of careful attention on the part of the observer, that Lussac was able to record the temporary inversion of the law of change of temperature above-mentioned.
Had he possessed modern instrumental equipment he would have brought down a yet more remarkable account of the upper regions which he visited, and learned that the variations of heat and cold were considerably more striking than he supposed.
With a specially devised instrument used with special precautions, the writer, as will be shown hereafter, has been able to prove that the temperature of the air, as traversed in the wayward course of a balloon, is probably far more variable and complex than has been recorded by most observers.
The exceptional height claimed to have been reached by Gay Lassac need not for a moment be questioned, and the fact that he did not experience the same personal inconvenience as has been complained of by mountain climbers at far less altitudes admits of ready explanation. The physical exertion demanded of the mountaineer is entirely absent in the case of an aeronaut who is sailing at perfect ease in a free balloon. Moreover, it must be remembered that--a most important consideration--the aerial voyager, necessarily travelling with the wind, is unconscious, save at exceptional moments, of any breeze whatever, and it is a well-established fact that a degree of cold which might be insupportable when a breeze is stirring may be but little felt in dead calm. It should also be remembered, in duly regarding Gay Lussac's remarkable record, that this was not his first experience of high altitudes, and it is an acknowledged truth that an aeronaut, especially if he be an enthusiast, quickly becomes acclimatised to his new element, and sufficiently inured to its occasional rigours.
