During their descent, when at 1,200 feet from the earth, a heavy fall of snow was encountered, accompanied by a sudden and very great reduction of temperature, the thermometer dropping to 22°, or 10° below freezing point. The mercury in the barometer at this moment had risen to nineteen inches.
I mention this circumstance for the purpose of showing that sometimes sudden changes of temperature have been experienced, not only by Green, but by Bixio and Barral later on in the present century.
The fatigue of the muscular powers, occasioned by exertion in emptying ballast, did not occasion any serious inconvenience in respect to difficulty in respiration.
We shall see, in the next ascent which was still higher, that the plan I have already exemplified as to allowing considerable space for expansion was resorted to, and this saved both the necessity for and the depression consequent upon hard work, although a large volume of gas was literally wasted, which might, in an economical point of view, have been prevented; but it will serve to show that a large balloon partially inflated, with a reduced amount of sand, is for all practical and scientific purposes preferable to a fully inflated balloon, that is, for very high ascents.
The ordinary way of examining the specific gravity of the different gases is by a simple method founded on the principles of pneumatics, for discovering the relative specific gravities of the aëriform fluids.
This consists in observing the time that a given portion of the gas, under a determined pressure, takes to escape through a very small aperture. The density of the gaseous fluid must be inversely as the square of the interval that elapses.
The weight of the balloon and all appendages must evidently compress the included gas, and thereby render it in some degree denser.
To compute this minute effect, we have only to consider that the pressure of a column of atmosphere at the mean temperature, and near the level of the sea, is 1632 pounds on a circle of a foot in diameter.
Thus, in a balloon of sixty feet in diameter, if we suppose the whole load to have been 6000 pounds, the compression of the bag would only amount to five-thirds of a pound for each circle of a foot in diameter in the horizontal action, or corresponding to the 979th part of the entire pressure of the atmosphere.
But the weight of the confined gas (hydrogen) being 1200 pounds, its buoyancy must have suffered a diminution of somewhat more than a pound or one-eleventh from the circumference opposed to it.