and the S

ombination of windlass and pulleys made it possible for a horse to lift a heavy bucket of coal. There came a time, however, when slow and crude methods such as these could not supply the coal as fast as it was needed. The shallower mines were bein

se coal with speed from the deepest mines. Then it happened that two great inventors, Newcomen and Watt, arose to produce the machine that was needed.

of the pressure of the air. There was no apparent connection between these experiments and the art of coal-mining, yet these discove

eam-Engine w

nd wanting. He next turned his attention to steam, and discovered that if the piston were forced up by steam and then the steam condensed, a vacuum was formed beneath the piston, and air-pressure forced the piston to descend. If the piston were attached to a weight by a rope passing over a pulley, then, as the piston descended, it would lift the weight. Papin's engine consisted simply of a cylinder and

–PAPIN'

ton B was forced down by air-pressure, a weight wa

mith by trade, and his great successor, Watt, was a mechanic. Thus we see that great discoveries soon be

men's

its own weight. The steam was condensed in the cylinder, and the pressure of the air forced the piston down. Thus the work of raising water in the pump was done by the air. Newcomen's first

work with unusual rapidity. The upper side of the piston was covered with water to make the piston air-tight, and it was found that this water was entering t

ey Potter, to whom this work was intrusted, won fame by tying strings to the cocks in such a way that the engine

form of valve-gear is the link motion invented by Stephenson. This form will be described in connection with the locomo

cost of running the engines increased. The largest engines consumed about $15,000 worth of coal per year. The Newcomen engine required about twenty-eight pounds of coal per hour per horse-p

n years, the Guild of Hammermen, a labor-union of his time, refused him admission, and this refusal meant no employment. He found sh

's E

nics had failed to make it work. The job was given to Watt. That he might do a perfect piece o

IN REPAIRING WHICH WATT WAS

the Universit

always as hot as the steam that enters it. While thinking upon this problem the idea came to him that, if connection were made between the cylinder and a tank from

act on both sides of the piston. So he proposed to put an air-tight cover on the cylinder with a hole and stuffing-box for the piston to slide through and to admit steam

m the cylinder when the piston had made about one-fourth of a stroke. The steam in the cylinder continues to e

DER OF WATT'S

the course o

wer of a

ondon horse could go on lifting 150 pounds over a pulley at the rate of 2-1/2 miles an hour or 220 feet per minute, and continue the work eight hours a day." This would be equal to lifting 33,000 pounds one foot high every minute. This rate of doing work he called a horse-power. It is more than the average horse can do, but t

inventions made by Watt (Fig. 15). The fly-ball governor replaced the throttle-valve which was at fir

FLY-BALL

admission of steam to the cylinder b

engine to locomotion on sea and land, to develop the steam-turbine, and so to increase the power of the steam-engine that,

Leyd

essor had tried what he called a new but terrible experiment. He had suspended by two silk threads a gun-barrel which received electricity from an electrical machine. From one end of the gun-barrel hung a brass wire. The lower end of this wire dipped in a j

r is simply a glass bottle or jar coated with tin-foil both inside and outside (Fig. 16). When charged with electricity the jar will hold its charge until the two coatings are connected by a

6–A LE

ecause of the shock felt on taking through the body the discharge from the "wonderful bottle," and the fact that several persons could receive the shock at the same instant. On one occasion the Abbé Nollet disch

rs and I

es electrified. Electricity can be transferred from one object to another. Gray discovered further that contact is not necessary, that a hempen thread or a wire will carry an electric charge from one object to another. A silk thread will not carry the electric charge. "Some things convey electricity," he said, "and some do not, and those which do not can be used to prevent the electricity escaping from th

of Elect

bjects on the gold leaf, he rubbed a piece of gum-copal and brought it near the leaf. To his astonishment the leaf, which was repelled by the glass tube, was attracted by the gum-copal. He repeated the experiment again and again, and each time the

this way he could get a much stronger spark than with a single jar. On one occasion he nearly lost his life by taking a shock from his

's Kite

nder-cloud, thinking electricity from the cloud would follow the string of the kite and could be stored in a Leyden jar, and used like the charge from an electrical machine. He had felt the power of a Leyden-jar discharge, and through it had nearly lost his life. He knew that lightning is far more powerful than any bat

e ribbon he held the cord to insulate it from his hand. The kite soared into the clouds, and Franklin and his son stood under a shed awaiting the coming of the "electric fire" (Fig. 17). Soon the fibres of the cord began to bristle up. He approached his knuckles to

NKLIN'S KIT

tricity fro

ightn

runs out silently at the point." In the dark you may see a light gather upon the point like that of a firefly or glow-worm. If the needle is held in the hand and brought near to an object charged with electricity, the object is quietly discharged, and a light may be seen at the po

the moist earth below the surface. The lightning-rod has not proved so great a protection as Franklin supposed it would. He supposed that a lightning-stroke is a discharge in one direction only; but we now know that it is a rapid surging back and forth, and this fact accounts for the failure of the lightning-rods to furnish perfect protecti

tricity to be a subtle fluid existing in all objects. If an object has more than a certain amount

o explain why two bodies negatively electrified should repel each other. According to Symmer, an uncharged body contains an equal quant

rified, and attracted each other so strongly that a force of about one pound was required to separate them. The two charges, negative and positive, could, however, be separated. He thought, therefore, that there are "two electrical powers," not one, as Franklin believed.

e gained possession of the field. Each contained some truth, and each had

d the Elec

icity of the atmosphere followed this discovery. Aloisio Galvani, a physician in Bologna, Italy, in attempting to learn the effect of atmospheric

of the thigh with the point of a knife while a spark was drawn from the electrical machine, the muscles contracted violently, as if they w

another long iron wire, which was allowed to dip in the water in the well. "The result," he said, "came about as we wished. As often as the lightning broke forth, the muscles were thrown into repeated violent convulsions, so that always, as the lightn

nal cord, fell into convulsions not only when it lightened, but when the sky was calm and clear, I thought that the cause of these contractions was the changes in the electricity of the atmosphere. Then for hours, yes, even days, I observed the animals, but almost never a movement of the muscles could be seen. At last, tired with such fruitless waiting, I bega

icity to pass through the muscles of the frog. Galvani did not know that he had discovered a new source of el

the Electr

, formed an electric battery. Volta showed that an electric charge can be produced merely by bringing two different metals into contact. He found

NG HIS ELECTRIC BATTER

ting. Photo

air of disks. With his column of disks he could obtain a strong shock; indeed, many shocks, one after the other. This first battery of Volta's was a form of "dry battery." Later

FIRST ELEC

ne hundred pairs of

uch batteri

Italian Institute of

current depends on the rate at which the metal is dissolved by the acid; but he had disco