89

Classification of the Sci.
89

sitting for two or three hours in a large clothes basket on the floor of one's study differs from that of a balloon ascension chiefly in there being more sense of motion in the former case and more of novelty to be seen. The balloon is best made of specially woven silk, varnished with a flexible varnish of which there are many kinds, mostly of secret composition. It is enclosed in a netting made of special small ropes like fish-line. The car is of wicker-work. The valve is the most studied part of the thing. Zeppelin's valve seems to be excellent. This sort of balloon can never be in equilibrium. It must rise rapidly enough not to be in danger of being thrown against trees. It therefore soon acquires a great upward velocity; say, of four or five hundred feet a second. At first, though the air becomes rarer in mounting, yet the gas swells in the same proportion, and until the balloon is entirely filled, the same mass of air is displaced by the same mass of gas; hence the upward acceleration continues and the balloon rises faster and faster. After the balloon is completely filled, as the air becomes rarer, the same proportion of gas escapes, and therefore the ascensional