We continue rising. The balloon will, of course, persist in doing so until the weight of the included gas and of the entire apparatus exactly balances an equal bulk of the surrounding air. Starting from the earth with all its buoyant power in hand, it would soon acquire a considerable momentum were it not controlled by the resistance of the atmosphere, which reduces its motion to a steady, uniform ascent. This presumes, however, that nothing transpires to alter its gravity. The addition of a few rain-drops to the machine would infallibly slacken its speed, whilst the fall overboard of one of the passengers would convert it for the time into a runaway balloon. When Mr. Cocking severed his parachute from the great Nassau, the latter, huge as it was, bounded aloft with such swiftness that whilst the poor fellow was descending to death, the two aeronauts seemed to be mounting to destruction, either by the bursting of the balloon or the stifling emission of gas.
In another way, also, too rapid a start may lead to dangerous consequences. In 1850, MM. Bixio and Barral took their places in the car of a balloon inflated with pure hydrogen. Their object in using this lightest of all aerial fluids was to climb to an elevation of thirty or forty thousand feet; but not having made due allowance for its buoyancy, the machine, when released, shot through the air like a ball from a gun. The envelope expanded so rapidly that it bulged down upon the aeronauts and shrouded them completely, the car being slung at too slight a distance below. Struggling like men beneath a fallen tent, one of them, in his endeavours to extricate himself, tore a hole in the great bag, from which the gas poured upon them, producing illness and threatening suffocation. Precipitately they began to sink, and it was only by tossing everything overboard that they succeeded in landing safely on the earth. They had traversed a bed of clouds 9,000 feet in thickness, reached a height of 19,000 feet, and then performed the return journey all in the space of little more than three quarters of an hour.
Higher and higher we mount. Shall not we knock our sublime heads against the stars, if we continue to ascend in this indefinite way? How rapidly we move, and what curious effects vertical travelling may involve, a single illustration will suggest. Aeronauts may enjoy a spectacle which, at the first mention, might almost recall the retrograde movement of the solar shadow on the dial of Ahaz—namely, that of two sunsets in one day. An early balloonist, M. Charles, was very much impressed by this vision. When he left the earth for an evening excursion, the great luminary had just disappeared, but, said the Frenchman, proudly, 'he rose again for me alone!' 'I had the pleasure of seeing him set twice on the same day.' For was the spectacle such as the dwellers on the soil may command, by permitting the orb to sink behind some elevation, and then mounting it so as to bring him again into view—thus playing at bo-peep with the lord of day. For, continued M. Charles, still more proudly, 'I was the only illuminated object; all the rest of nature being plunged into shadow!'
But now, looking aloft, we observe a mass of clouds, towards which we are rapidly speeding. There are mountains of snow and great threatening rocks, against which it seems as if our fragile vessel would inevitably be dashed. The novice in aerial navigation almost instinctively holds his breath as he sees the distance narrowing between his frail skiff and these frowning piles, and awaits the awful collision. But they open as if by magic, and the balloon glides into the midst without a shock, or a tremor in its frame. We are then enveloped for a time in a sort of obscurity, but we have nothing to fear, for the machine might travel blindfold without dread of the slightest obstruction in these pathless expanses. Destitute of every object which could serve as a guide, we proceed until we emerge into sunshine once more, and then, looking down, we see the clouds through which we have entered closing like a trap-door after us, and shutting us out from the dear old world, where we lead such a life of charmed misery.
Sometimes, however, it seems impossible to rise above the 'smoke and stir of this dim spot, which men call earth.'
In an ascent from Wolverton, in June, 1863, Mr. Glaisher passed through an extraordinary succession of fogs and showers and rain-clouds; and though he soared to a height of 23,000 feet, the balloon was unable to extricate itself from its earthly entanglements. Following a fine rain came a dry fog, which continued for some distance; this traversed, the aeronauts entered a wetting fog, and subsequently a dry one again. When three miles in height, they imagined that they would certainly break through the clouds, but, to their great surprise, nebulous heaps lay above them, beneath them, and all around them. Up they clambered, but at an elevation of four miles dense masses still hung overhead as if to forbid any further progress, and two clouds with fringed edges specially attracted their attention, from the fact that they were unmistakeably nimbi, although formations of this latter class are mostly creatures of the nether sky. On returning, a heavy rain fell pattering on the balloon at an altitude of three miles, and then, lower down, for a space of 5,000 feet, they passed through a curious snowy discharge, the air being full of icy crystals, though the season was high summer.
It is not often, however, that the atmosphere is in this nebulous condition throughout so large a portion of its depth. For days together terrestrials may be enveloped in fog and rain, and in that case must wait patiently until the clouds please to roll off, and drench some other locality; but if at such seasons we were to jump into a balloon, we might soon pass out of the watery zone and soar into the jocund sunshine. Continuing our ascent, therefore, through the dense tract of moisture we first entered, our machine at last lifts its head joyously above the surface, and shaking off the cloudy spray, bounds into a new sphere, where the great giver of light glows with unadulterated ray. We are, in fact, in a new world. We are completely cut off from our native earth by a huge continent of vapour, which appears to have been suddenly petrified into rock.
'Above our heads,' writes Mr. Glaisher, 'rises a noble roof, a vast dome of the deepest blue. In the east may perhaps be seen the tints of a rainbow on the point of vanishing; in the west, the sun silvering the edges of broken clouds. Below these light vapours may rise a chain of mountains, the Alps of the sky, rearing themselves one above the other, mountain above mountain, till the highest peaks are coloured by the setting sun. Some of these compact masses look as if ravaged by avalanches, or rent by the irresistible movement of glaciers. Some clouds seem built up of quartz, or even diamonds: some, like immense cones, boldly rise upwards; others resemble pyramids whose sides are in rough outline. These scenes are so varied and beautiful that we feel we could remain for ever to wander above these boundless plains.'
As we ascend, however, a serious question comes into play. To the first adventurer we may suppose that it would present itself with alarming force. Shall we be able to breathe safely in yonder upper regions, where the air is so thin that the lungs must work 'double shift,' as it were, to procure their necessary supply? At the earth's surface, it is well known that the atmosphere presses upon every square inch with a force of from fourteen to fifteen pounds. A column of air forty miles in height resting upon a man's hat, would, of course, crush it flat upon his head in a moment, were it not for an equal resistance within; and, but for the same cause (the equal diffusion of pressure at the same level), we should all go staggering along under our burden of thirty thousand pounds—such is our share of the atmospheric load—or, if laid prostrate, should find ourselves incapable of rising. But of course the pressure grows smaller as we ascend, for the simple reason that the height of the column above us continually decreases. Seeing, moreover, that we are adapted by our organization to existence at the bottom of this aerial ocean, it is natural to expect that at considerable elevations some sensible disturbance of our functions will ensue. At the height of three miles and three-quarters the barometer, which stands at about thirty inches at the level of the sea, has sunk to fifteen inches, exhibiting a pressure of some seven-and-a-half pounds to the square inch, and showing that as much of the atmosphere in weight is below us as there is above. Reaching an elevation of between five and six miles, the mercury would be found to mark ten inches only, representing a pressure of five pounds to the square inch, and proving that two-thirds of the aerial ocean had been surmounted, leaving a thin third alone to be traversed. The following table, as given by Mr. Glaisher, will, however, best express this decline of density:—
'At the height of 1 mile the barometer reading is 24·7 in.
" 2 miles""20·3 "
" 3 miles""16·7 "
" 4 miles""13·7 "
" 5 miles""11·3 "
"10 miles"" 4·7 "
"15 miles"" 1·6 "
"20 miles"" 1·0 " less.'