Part 11 (1/2)
[Ill.u.s.tration: FIG. 85--DE LAVAL STEAM-TURBINE Driven by a jet of steam striking the blades.]
[Ill.u.s.tration: FIG. 86--A MODERN STEAM-TURBINE WITH TOP CASING RAISED SHOWING BLADES]
[Ill.u.s.tration: FIG. 87--DIAGRAM OF TURBINE SHOWN IN FIG. 86 The arrows show the course of the steam.]
[Ill.u.s.tration: FIG. 88--A STEAM-TURBINE THAT RUNS A DYNAMO GENERATING 14,000 ELECTRICAL HORSE-POWER The steam enters through the large pipe at the left.]
In 1897, as the battle-s.h.i.+ps of the British fleet were a.s.sembled to celebrate the Diamond jubilee of Queen Victoria, a little vessel a hundred feet long darted in and out among the giant s.h.i.+ps, defied the patrol-boats whose duty it was to keep out intruders, and raced down the lines of battle-s.h.i.+ps at the then unheard-of speed of thirty-five knots an hour. It was the _Turbinia_, fitted with the Parsons turbine. This event marked the beginning of the modern turbine. It also marked the beginning of a revolution in steam propulsion.
The Parsons turbine does not use the jet method, but the steam enters near the centre of the wheel and flows toward the rim, pa.s.sing over a number of rows of curved blades. The Parsons turbine is used on the fastest ocean liners. The _Lusitania_, one of the fastest steams.h.i.+ps in the first decade of the twentieth century, has two sets of high and low pressure turbines with a total of 68,000 horse-power.
The windmill is a form of turbine driven by the air. As the air rushes against the blades of the windmill, it forces them to turn. If the windmill were turned by some mechanical power, it would drive the air back, and we should have a blower. This is what we have in the electric fan, a small windmill driven by an electric motor so that it drives the air instead of being driven by it. The blades of the windmill and the electric fan are shaped very much like the screw propeller. The screw propeller, driven by an engine, would drive the water back if the s.h.i.+p were firmly anch.o.r.ed, just as the fan drives the air. But it cannot drive the water back without pus.h.i.+ng forward on the s.h.i.+p at the same time, and this forward push propels the s.h.i.+p. It is difficult to attain what is now regarded as high speed with a single screw. With engines in pairs and two lines of shafting higher power can be used. The best steamers, therefore, are fitted with the twin-screw propeller. Some large steamers have three and some four screws.
The screw propellers of turbine steams.h.i.+ps are made of small diameter, that they may rotate at high speed without undue waste of power. By the use of turbine engines and twin-screw propellers, the weight of the machinery has been greatly reduced. The old paddle-wheels, with low-pressure engines, developed only about two horse-power for each ton of machinery. The turbine, with the twin-screw propeller, develops from six to seven horse-power for every ton of machinery. The modern steamer, with all its machinery and coal for an Atlantic voyage, weighs no more than the engines of the old paddle-wheel type and coal would weigh for the same horse-power. The steam-turbine and the twin-screw propeller have made rapid ocean travel possible.
Chapter VI
THE TWENTIETH-CENTURY OUTLOOK
We have seen that the latter half of the nineteenth century was a time of invention. It was a time when the great discoveries of many centuries bore fruit in great inventions. It was thought by some scientists that all the great discoveries had been made, and that all that remained was careful work in applying the great principles that had been discovered.
So far was this from being true that in the last ten years of the nineteenth century discoveries were made more startling, if possible, than any that had preceded. The nineteenth century not only brought forth many great inventions, but handed down to the twentieth century a series of discoveries that point the way to still greater inventions.
Air-s.h.i.+ps
For centuries men sailed over the water at the mercy of the wind. The sailing vessel is helpless in a storm. Early in the nineteenth century they learned to use the power of steam for ocean travel, and the wind lost its terrors. Late in the eighteenth century men learned to sail through the air in balloons even more at the mercy of the wind than the sailing vessels on the ocean. More than a hundred years later they learned to propel air-s.h.i.+ps in the teeth of the wind. The nineteenth century saw the mastery of the water. The twentieth is witnessing the mastery of the air.
The first balloon ascension was made in 1783, two men being carried over Paris by what Benjamin Franklin called a ”bag of smoke.” The balloon was a bag of oiled silk open at the bottom. In the middle of the opening was a grate in which bundles of f.a.gots and sheaves of straw were burned. The heated air filled the balloon, and as the heated air was lighter than the air around it the balloon could rise and carry a load. Beneath the grate was a wicker car for the men. They were supplied with straw and f.a.gots with which to feed the fire. When they wanted to rise higher they added fuel to heat the air in the balloon. When they wished to descend they allowed the fire to die out, so that the air in the balloon would cool. They could not guide the balloon, but drifted with the wind. That great philosopher Benjamin Franklin, who saw the ascension, said that the time might come when the balloon could be made to move in a calm and guided in a wind. In the second ascension bags of sand were taken as ballast, and the car was suspended from a net which enclosed the balloon. In this second ascension hydrogen gas was used in place of heated air.
The greatest height ever reached by a human being is about seven miles.
This height was first reached in 1862 by two balloonists who nearly lost their lives in the adventure. At a height of nearly six miles one of the men became unconscious. The other tried to pull the valve-cord to allow the gas to escape, but found that the cord was out of his reach. His hands were frozen, but he climbed out of the car into the netting of the balloon, secured the cord in his teeth, returned to the car, and threw the weight of his body on the cord. This opened the valve and the balloon descended.
Those who go to great heights now provide themselves with tanks of compressed oxygen. Then when the air becomes so thin and rare that breathing is difficult they can breathe from the oxygen tanks.
A captive balloon in war serves as an observation tower from which to observe the enemy. It is connected to the ground by a cable. This cable is wound on a drum carried by the balloon wagon. The balloon can be lowered or raised by winding or unwinding the cable.
The gas-bag is sometimes made of oiled silk, sometimes of two layers of cotton cloth with vulcanized rubber between. The cotton cloth gives the strength needed, and the rubber makes the bag gas-tight.
The most convenient gas for filling balloons is heated air, but the air cools rapidly and loses its lifting power. Coal-gas furnished by city gas-plants is sometimes used. This gas will lift about thirty-five pounds for every thousand cubic feet. A balloon holding thirty-five thousand cubic feet of coal gas will easily lift the car and three persons. The lightest gas is hydrogen. This gas will lift about seventy pounds for every thousand cubic feet. Hydrogen is made by the action of sulphuric acid and water on iron. If a bit of iron is thrown into a mixture of sulphuric acid and water bubbles of hydrogen gas will rise through the liquid. This gas will burn if a lighted match is brought near.
A balloon without propelling or steering apparatus is not an air-s.h.i.+p.
It may be raised by throwing out ballast or lowered by letting out gas, but further than this the aeronaut has no control over its movements.
The balloon moves with the wind. No breeze is felt, for balloon and air move together. To the aeronaut the balloon seems to be in a dead calm.
It is only when he catches sight of houses and trees and rivers darting past below that he realizes that the balloon is moving.
If a balloon has a propelling apparatus it may move against the wind, or it may outspeed the wind. A balloon with propelling and steering apparatus is called a ”dirigible” balloon, which means a balloon that can be guided. Figs. 89 and 90 are from photographs of a ”dirigible”
used in the British army. Such a balloon is usually long and pointed like a spindle or a cigar. It is built to cut the air, just as a rowboat built for speed is long and pointed so that it may cut the water. The propeller acts like an electric fan. An electric fan drives the air before it, but the air pushes back on the fan just as much as the fan pushes forward on the air, and if the fan were suspended by a long cord it would move backward. So the large fan or screw propeller on an air-s.h.i.+p drives the air backward, and the air reacts and drives the s.h.i.+p forward. In the same way the screw-propeller of an ocean liner drives the vessel forward by the reaction of the water.