The air has weight,—yes, more than you have been accustomed to think, perhaps. Just take the pen or pencil and mark out a piece of paper one square inch. Having marked out this, you are to understand that the air presses upon it with a force of about fifteen pounds. I say about, for the reason that atmospheric pressure is not always exactly the same. Sometimes it presses rather more, at other times rather less than fifteen pounds, but still the variation is not great.
Thus, air has weight, and you also know that air, being a fluid, permits objects to move about in it, so it follows that if we can find anything number two lighter than the air, it will ascend in the air. We shall look in vain amongst liquids and solids for thing number two, all being heavier than air; but of gases lighter than air there exist several. Why not use them? We can use some of them, and we presently will; but it stands to reason that we can no more use a gas for lifting a weight up into air without first putting the gas into a bag, than we can use a horse for carriage-draught without first putting a collar round his neck, then attaching shaft and traces: no, indeed, not so well even, because in a case of emergency a carriage might be fixed to the horse’s tail!
The very simplest form of balloon is a soap-bubble. Most of us have probably at some time or other amused ourselves by blowing soap-bubbles, without perhaps troubling ourselves to understand the whys and wherefores of the case.
Why is it that a soap-bubble blown with air from the lungs ascends? You will tell me perhaps that it ascends because the air which comes from the lungs is of a lighter ‘nature’ than the atmosphere No, indeed, instead of having a lighter it has a heavier nature. Though air which comes out of the lungs is by ‘nature’ heavier than air which goes into the lungs, yet at the same time it is warmer, and for the reason of that warmth it remains during the existence of the warmth practically lighter.
This circumstance being remembered, you will at once understand why it is that a soap-bubble, even if it lives—so to put the case—long enough, does not continue to ascend as a balloon would have continued, but first slackens in its upward course, then descends, the reason of descent being the cooling of the air within it. If you had been clever enough to measure that soap-bubble twice, first at the very moment of rising, next at the last moment of setting, you would have found the warmer bubble to have been the larger.
If you were to blow a thin bag full of air, you would not expect it to ascend, on the supposition, that is to say, that the bagged air and the free air are of one and the same temperature; but if you could manage to make the bagged air hot, and keep it hot, then you would get lifting power. This is exactly what is done in the Montgolfier, or fire-balloon.
The reason of this seems very simple when one comes to think about it, yet the Brothers Montgolfier, inventors of the fire-balloon, did not exactly see the true science of the balloon that bears their name. They appeared to have reasoned somewhat in the following manner. A wreath of smoke ascends, therefore smoke must be lighter than air; therefore a bag filled with smoke, and kept full of smoke, should also ascend, supposing the bag to be made of sufficiently light materials.
If the Montgolfiers had said, instead of smoke, hot air, they would have truly represented the fact.
They made a bag of canvas, and they lined it with paper. Under the neck of this bag they hung a sort of fire-grate, and underneath this, but within reach of it, a car. Impressed as they were with the smoky notion, one of their first cares was to feed the fireplace with some variety of fuel that would yield abundant smoke. Chopped straw, slightly damped and mixed with wool, was one of the favoured combustibles.
Fire, or Montgolfier balloons, large enough to take up sky travellers, are no longer used; air-balloons, or, as it would be more proper to call them, gas-balloons, being more convenient and at the same time more safe. Toy fire-balloons are, however, still made and let off as a pretty firework, and I shall now describe the best and easiest ways of making them.