We proceed. We are now cruising in the full glare of the sun. The rays of that luminary beat upon us with scorching force; but whilst the head seems to be in the Sahara, the feet may be in Spitzbergen. For here, as on the top of a snow-clad mountain, the temperature of the air is one thing, the direct heat of the sun is quite another. The difference may amount to thirty or forty degrees in an ordinary ascent, and of course, becomes more noticeable the higher the flight. The thin air and scanty vapour of the upper regions furnish us with flimsy clothing; whilst in the nether world we wrap the dense medium round us like a mantle, and keep our caloric within our frames.
Is there any law, however, by which the decrease of temperature can be expressed? Seeing that the atmosphere is divided, as it were, into various storeys, these being formed of changing currents, or fugitive strata of clouds, each with its peculiar charge of heat, is it possible that any fixed principle of decline can be detected?
Take a few results. On leaving the ground, where the temperature was 50° (in the afternoon of the 31st of March, 1863), the thermometer indicated 33½° at one mile, 26° at two miles, 14° at three miles, 8° at 3¾ miles, where a bed of air heated to 12° was entered, and then at an elevation of 4½ miles, the instrument had fallen to zero. In descending, the temperature rose to 11° at about three miles in height, it sank to 7° in passing a cold layer, afterwards increased to 18½° at two miles, to 25½° at one mile, and finally settled at 42° on the ground.
Again, on starting (17th July, 1862), the temperature at the surface was 59°, at 4,000 feet, it was 45°, and at 10,000 feet it had sunk to 26°. For the next 3,000 feet it remained stationary, during which time the aeronauts donned additional clothing, in anticipation of a severe interview with the Frost King; but to their great surprise, the thermometer rose to 31° at 15,500 feet, and to 42° at 19,500 feet, by which time they found it necessary to divest themselves of their winter habiliments. Sometimes, indeed, the changes of temperature experienced are startling and unaccountable. At an elevation of 20,000 feet, Barral and Bixio, whilst enveloped in a cloud, found their thermometer at 15° Fahr. Above this cloud, at a height of 23,127 feet, the instrument had sunk to 38° below zero, making a difference of not less than 54° of heat between the two points. Judging from this observation, might we not expect to find all the moisture at those cheerless altitudes curdled into ice? and if our globe is sheathed in an envelope of frozen particles, is the fact wholly without meaning in reference to the aurora and other meteorological phenomena?
From such capricious data, it would seem impossible to extract any definite law; but it has been assumed by many that, taking all things into account, the temperature decreases one degree for every 300 feet of elevation. Putting the matter more exactly, there is, according to Flammarion, a mean abatement of one degree for every 345 feet where the sky is clear, and of one degree for every 354 feet when the heavens are overcast; the decline being quicker when the day is hot than when it is cold, and in the evening than in the morning. Mr. Glaisher, however, feels himself compelled to repudiate this theory of a steady, constant diminution of heat. The results of all his midday experiments amounted to this:—
'The change from the ground to 1,000 feet high was 4° 5´ with a cloudy sky, and 6° 2´ with a clear sky. At 10,000 feet high it was 2° 2´ with a cloudy sky, and 2° with a clear sky. At 20,000 feet high the decline of temperature was 1° 1´ with a cloudy sky, and 1° 2´ with a clear sky. At 30,000 feet the whole decline of temperature was found to be 62°. Within the first 1,000 feet the average space passed through for 1° was 223 feet with a cloudy sky, and 162 feet with a clear sky. At 10,000 feet the space passed through for a like decline was 455 feet for the former, and 417 feet for the latter; and above 20,000 feet high the space with both states of the sky was 1,000 feet nearly for a decline of 1°. As regards the law just indicated, it is far more natural and far more consistent than that of a uniform rate of decrease.'
It should be carefully observed that these conclusions refer to ascents by day; and that by night the temperature augments within certain limits, as Marcet showed, and as numerous experiments have confirmed.
Scarcely less interesting is the question as to the moisture in the atmosphere. Does it decline according to any graduated law? From a large number of observations it has been concluded that the watery vapour increases up to a certain elevation (varying with the season of the year, the hour of the day, and the condition of the sky), and then, having reached this maximum, we find that the air grows continually drier the further we climb. Upon this simple fact much of the physical happiness of our globe depends, for it is the moisture in the lower regions which arrests the efflux of caloric, preserves it for home consumption, and assists the earth in the kindly production of its fruits.
Meanwhile, the rays of the sun playing with unchecked fervour upon the balloon, have been heating and expanding the gas. Lightened also by the dissipation of the moisture contracted in the cloudier portion of the ascent, it probably occurs to the voyager, particularly if he is prone to take alarming views of events, that as the machine rises into a rarer atmosphere the envelope may distend until it actually bursts. Nor is this apprehension, however painful to the nerves, wholly without foundation. Looking up at the flimsy globe above his head, he will observe that it is now fully inflated, though purposely left somewhat flaccid when the journey commenced; and, possibly, he may observe signs of the sun's action on its sides, as if it were blistering under the solar beams. Brioschi, the Neapolitan astronomer, wishing to soar higher than Gay-Lussac, who had reached 23,000 feet on his way to the stars, was stopped on his ambitious flight, as Icarus had been before him, by getting too near the sun. He had no wings to melt, it is true, but he had a balloon to rupture, and the swollen tissue accordingly gave way, though, happily, without involving him in the fate of the presumptuous youth. Will it be credited, however, that any aeronaut could deliberately make an ascent with the express intention of bursting his balloon himself? Yet this has been done without pre-engaging a coroner, and without the slightest wish to commit scientific suicide. The individual by whom this perilous experiment was performed was Mr. Wise, the American. He argued that if the explosion were neatly managed, the collapsing envelope would act as a sort of parachute, the lower part retreating into the upper, and forming a concavity which would present sufficient resistance to ensure a safe and steady descent. Nor were his expectations wholly disappointed. Having risen through a thunderstorm to a height of 13,000 feet, he fired his magazine of hydrogen gas. The car rushed down with awful rapidity, supported, however, by the relics, like a torn umbrella, and alighted upon the ground without inflicting any great violence upon the daring navigator. Not many weeks afterwards, he repeated the exploit, if such it may be called, and in exploding the gas tore the silk receptacle from top to bottom; but, with equal good fortune, he arrived at the earth without a broken limb, the machine having taken a spiral course in falling, which enabled him to descend with uniform velocity.
Having now reached the highest point to which our aerostat will mount so long as its weight continues unchanged, we surrender ourselves to the guidance of the current in which we are involved. In rising to a moderate elevation, a balloon will sometimes shoot through more than one of these aerial streams. Mr. Foster detected the existence of four distinct currents in one experiment, namely, from the E.N.E., N., S.W., and S.S.E., and on the following day found there were three, namely, from the E.N.E., S.E., and S.S.W. Sometimes an upper and an under current may move in opposite directions. Had it not been for this fact, M. Tissandier's début in the clouds might have terminated in his death in the ocean. Ascending with M. Duruof from Calais under somewhat rash and defiant circumstances, their balloon was borne out to sea, not towards the English coast, which might, perhaps, have been reached, but right up the North Sea, where they would probably have perished. Fortunately, after proceeding for some distance, they observed a fleet of cumuli steering for Calais at a depth of some 3,000 feet below, and by dropping into this counter stream they were floated back to land.