When the waves are urged against the tunnel by the wind, the water is dashed into it. Being partially checked by the friction, which acts exactly like the water that is checked by the Ram, the wave hurls itself up the channel, and flies out in showers of spray, high above the level of the original wave which caused it.
In the illustration are shown the Water-ram with its globular valve, and the safety or escape valve of the waste water. On the left is shown one of the natural Spout-holes, with the water dashing through its tunnel into a mass of spray.
USEFUL ARTS.
CHAPTER XI.
AËROSTATICS.—WEIGHT OF AIR.—EXPANSION BY HEAT.
Ascent and Descent.—The Balloon and the Parachute.—Description of the Balloon.—The Montgolfier Balloon.—Causes of its Abandonment.—The Gas Balloon.—Hydrogen Gas and its Manufacture.—The Gossamer Spider.—Reasons of its Ascent and Descent.—Many Species of Gossamers.—Description of the Parachute.—Its Mode of Action.—A Balloon converted into a Parachute.—Toy Parachutes.—Natural Parachutes.—The Dandelion Seed and its Structure.—The Flying Squirrel.—The Flying Monkey.—Flying Mice and Flying Opossums.—The Flying Dragon and its Pseudo-wings.—The Flying Frog.—Weight of Air.—Pressure per Square Inch.—The Air Ocean and its Storms.—Principle of Air-currents.—The Sun, the Earth, and the Air.—Ventilation of Mines.—Choke-damp and Fire-damp.—The Air-shafts.—Chimneys of Factories.—The Steam-blast.—The Barometer, and Mode of its Construction.—Water and Mercury.—Sucking Eggs and Sugar-cane.—Expansion of Water and Metals by Heat.—The Thermometer.—Wheel-making.
Aërostatics.
WE will begin this chapter with the only two modes at present known by which man can ascend from the earth or descend to it with safety, namely, the Balloon and the Parachute, the latter being generally attached to the former, and detachable at pleasure.
The Balloon is, in fact, as its name imports, a large, hollow, air-tight ball, filled with some substance lighter than ordinary air. The original Balloons by Montgolfier were filled with heated air exactly like our toy fire-balloons. Just as the supply of hot air is kept up in them by a sponge dipped in lighted spirits of wine, so in Montgolfier’s balloons the same object was attained by straw which was kept continually burning in a grate.
There were, however, two disadvantages about this plan. The first was the great danger of fire, which on one occasion did ignite a balloon when at a great height. The second was the perpetual labour required in keeping the fire alight. Straw burns very rapidly, and so the aëronaut had no opportunity of making those meteorologic observations in which consist almost the entire value of the balloon.
Then it was thought that hydrogen gas, being about fourteen times lighter than ordinary air, would answer the purpose, and such has proved to be the case. Formerly the gas was made at great expense from sulphuric acid and zinc, but it is now found that the common coal-gas is quite as efficient, very much cheaper, and fills the balloon much more rapidly.