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. CHAPTER V. SOME FAMOUS EARLY VOYAGERS. During certain years which now follow it will possibly be thought that our history, so far as incidents of special interest are concerned, somewhat languishes. Yet it may be wrong to regard this period as one of stagnation or retrogression.