manent galvanic power was needed; and this was not supplied till 1836, when Daniell brought out his constant battery. The appearance of this battery and the improved electro-magnets prepared by Prof. Henry, was followed in 1837 by the invention of apparatus for transmitting and recording communications, by Samuel F. B. Morse, who had been experimenting on the subject for five years. Application was at once made to the Congress of the United States for aid to construct a line of sufficient length to test the invention; and after discouraging delays, in 1843, the sum of $30,000 was appropriated by that body, with which a line was established between Baltimore and Washington, a distance of forty miles. The enterprise was crowned with complete success; and the first news transmitted was the proceedings of the democratic convention of 1844, then sitting in Baltimore, by which James K. Polk was nominated for the presidency.

So manifold were the advantages of telegraphic communication, that immediately on the announcement of Morse's success companies were formed, and wires were soon seen threading the country in all directions. The various lines now in operation in the United States and British Provinces make a total of about 45,000 miles, on nine-tenths of which Morse's apparatus is used, House's and Bain's being chiefly employed on the remainder. With Morse's instruments about 9,000 letters may be transmitted in an hour. The construction of the line costs not far from $150 a mile.

The same year in which Morse perfected his invention (1837), plans for telegraphic communication based on the deflections of the needle were announced by Wheatstone in England, and Steinheil [stine'-hile], a German philosopher, to whom the discovery that the earth could be made to complete the circuit seems to be due. They are therefore sometimes mentioned as entitled to share with Morse the honor of his great invention. Their systems, however, were but modifications of what had been proposed some years before; though practicable, they could not compete in rapidity of operation with Morse's, and consequently never came into general use.

923. ELECTRO-MAGNETIC CLOCKS.-American ingenuity

forth by the discovery of electro-magnetism? By whom and when was the first perfect apparatus for transmitting and recording communications invented? What two improvements prepared the way for Morse's invention? How was Morse enabled to test his invention? What was the result? What was the first news transmitted? How many miles of telegraph are now in operation? On how much of this is Morse's apparatus used? What is the cost of constructing a telegraphic line? Who are sometimes mentioned as sharing with Morse the honor of inventing the telegraph?

ELECTRO-MAGNETIC CLOCKS.

365

has applied electro-magnetism to the determining of minute intervals of time and the regulation of clocks. The time of astronomical observations may thus be fixed with perfect precision to the tenth of a second.

The pendulum of a clock, for instance, is, by some mechanical contrivance, made by its vibrations to close and break a galvanic circuit. With Morse's apparatus, each vibration is indicated by a dot on a strip of paper passed in front of the style. If now an observer have a signal-key connected with the same circuit, by depressing it the instant a star passes one of the wires of his telescope, he permanently records its transit on the same paper by a dot intermediate between two vibration-dots, the exact time of which is known.

924. By the same agency a number of clocks may be made to keep uniform time.

This is effected by connecting any number of distant clocks, by means of wires, with one standard time-piece, which is itself connected with a galvanic battery,—so that the circuit may be closed and broken by all the pendulums simultaneously. Wheels connect the pendulums with the hands of the clocks, which are thus made to move with perfect uniformity. Some railroad companies use an arrangement of this kind to make the clocks at their different stations keep time together.

925. ELECTRO-MAGNETIC FIRE-ALARM.-The principle of the telegraph has been used for raising a simultaneous alarm of fire at a number of different stations connected with one principal station by wires. By completing and breaking the galvanic circuit, an attendant who is constantly on watch at the principal station, and receives his information by telegraphic signals from the district in which the fire is detected, strikes alarm-bells at the various distant stations a certain number of times, according to the number of the district in question. Such an arrangement has been used in Boston with great success.

926. THE HELIX, A MAGNET.-The helix, when traversed by a current of electricity, not only has high magnetizing powers, as we have seen, but is also itself a magnet. If

What is said of their claims? 923. To what has American ingenuity applied electromagnetism? Show how an astronomical observation may be telegraphically recorded. 924. How may a number of clocks be made to keep uniform time by means of electro-magnetism? 925. For what has the principle of the telegraph been used? Show how an alarm of fire may be simultaneously raised at different stations. 926. What is the effect of an electric current traversing a helix on the helix itself?

suspended so as to allow it freedom of motion, it points north and south, and dips like the magnetic needle. So, like poles of two helices repel each other; unlike poles attract each other.

Even when not bent in the form of helices, two wires traversed by electric currents, if brought near each other in parallel lines and free to move, exhibit mutual attraction or repulsion. When their currents move in the same direction, they attract each other; when in contrary directions, they repel each other.

Magneto-electricity.

927. Not only is magnetism developed by electric currents, but electric currents are produced by magnetism. That branch of science which treats of electric currents so produced is called Magneto-electricity.

The phenomena of magneto-electricity, like those of electro-magnetism, go far towards proving the intimate connection between electricity and magnetism, if not their actual identity.

928. Experiments.—Connect the ends of wire from a helix with a galvanometer. Then quickly thrust into the helix one of the poles of a bar magnet. The needle of the galvanometer is at once deflected, showing the passage of an electric current over the wire. If the opposite pole is introduced into the helix, a current passes in the contrary direction.

Within a helix place a soft iron bar of such length that each end may project a little. Over its ends bring the poles of a horse-shoe magnet, so suspended as to have freedom of revolution. On turning the magnet rapidly, the poles of the bar are reversed twice for each revolution, and an electric current is produced on the wire, as is shown by a galvanometer attached to it. This principle has been applied in different magneto-electric machines, with which water may be decomposed, platinum wire heated to redness, sparks produced, shocks given, and other experiments performed.

929. THE MAGNETO-ELECTRIC MACHINE.-Fig. 328 represents one form of the Electro-magnetic Machine.

S is a compound horse-shoe magnet supported on three pillars. In front of its poles, and as near as it can be brought without touching, is a bar of soft iron bent at right angles, and surrounded with several coils of insulated copper wire. The ends of this wire are pressed by springs against a con

Prove that it renders the helix magnetic. What phenomena are exhibited by two straight wires traversed by electric currents, when brought near each other? 927. What is Magneto-electricity? What is said of its phenomena? 928. What is the first experiment illustrative of magneto-electricity? The second experiment? In what is the principle here described applied? 929. Describe the Magneto-electric

ducting metallic plate, connected by wires passing under the stand with the screw-cups A, B. The soft iron armature just described is mounted on an axis which is made to revolve by a wheel turned by a handle. The handle being rapidly turned, each half-revolution of the armature brings its extremities near opposite poles of the magnet, thus reversing its polarity, and producing a strong electric current on the wire. If small copper cylinders attached to the wires are grasped one in each hand, as shown in the figure, a series of severe shocks are received, and the muscles are so contracted that it is almost impossible to open the hands and let go the conductors.

Machines of this kind, adapted to medical use, have been found efficacious in cases of rheumatism, dyspepsia, sprains, nervous diseases, &c., the current being made to pass through the diseased part.

Diamagnetism.

930. Experiments with powerful electro-magnets show that almost all substances are susceptible of magnetic influence. Some are attracted by the magnet; others, repelled; while a few are not acted on at all, though when more powerful magnets shall be made they may perhaps be found to fall under one of the two previous classes.

Hence arises a three-fold division of bodies. 1. Magnetic bodies, or such as are attracted by an electro-magnet.

Machine represented in Fig. 328, and its mode of operation. What is the effect of such a machine on the human system? What use has been made of machines of this kind? 930. What has been shown by experiments with powerful electro-magnets? Name the three classes into which bodies are divided with reference to the influence

2. Diamagnetic, or such as are repelled. 3. Indifferent, or such as are not acted on at all.

Fig. 329.

NI

I

B

The difference between these three classes of bodies may be illustrated with the apparatus shown in Fig. 329. N, S, are the poles of an electro-magnet, which is connected by the wires C, Z, with a galvanic battery. A bar of iron, nickel, cobalt, manganese, or other magnetic substance, suspended between the poles so as to move freely, will come to rest with its ends as near them as possible, in the position II. On the contrary, a bar of bismuth, phosphorus, zinc, tin, or other diamagnetic substance, similarly suspended, will be repelled and come to rest at right angles to the position just described, as shown by the dotted line,—with its sides opposite the poles of the axis and its ends as far from them as possible. Similar attraction and repulsion are exhibited if the substances are presented to either pole separately. An indifferent substance will remain in any position in which it is placed, being neither attracted like the iron nor repelled like the bismuth.

Similar experiments may be made on liquids and gases by enclosing them in tubes. It is thus found that oxygen is magnetic; water, alcohol, ether, and the oils, diamagnetic.

CHAPTER XVIII.

ASTRONOMY.

931. ASTRONOMY is the science that treats of the heavenly bodies, their motions, size, distance, &c.

By the heavenly bodies are meant the sun, the moon, stars, planets, and comets.

932. Astronomy, as it is the most sublime, is also the oldest of sciences. The shepherds of the patriarchal age, tending their flocks by day and night beneath the canopy of heaven, naturally directed their gaze to the brilliant

exerted on them by electro-magnets. Define each. Illustrate the difference between these three classes with the apparatus represented in Fig. 329. How may similar experiments be made on liquids and gases? What gas is found to be magnetic? What liquids are diamagnetic?

931. What is Astronomy? What are meant by the heavenly bodies? 932. Who