what we call the noise of the report. Since sound travels at about the rate of a mile in five seconds and the lightning discharge is practically instantaneous, the noise from different parts of the discharge will reach us at different times and to this and the echoing from clouds or hills is due the roll of the thunder. The distance of the flash can be told approximately by dividing the number of seconds between seeing the flash and hearing the thunder by five.
Frequently in the evening flashes called heat lightning are seen near the horizon. These are due to the reflection on clouds of flashes of lightning in a storm which is below the horizon. Thunder-storms occur sometimes in winter. They are very prevalent in the tropics.
81.Electrical Communication.
Experiment 87. -Attach one end of a wire to a pole of a dry cell and the other end to one of the binding posts of a telegraphic sounder. From the other binding post of the sounder lead a wire to the binding post of atelegraphic key. Connect the free binding post of the key withthe free pole of the battery (Fig. 76). When the key is pushed down, the circuit is closed and the sounder clicks. If a relay can be procured, remove the sounder and connect two of the binding posts of the relay in the same way that the sounder was connected.
Connect one of the free binding posts of the relay with a binding post of the sounder and the otherFig. 76.
binding post with the pole of a dry cell. Connect theother pole of the dry cell with the free binding post of the sounder. When the key closes the circuit through the relay, the circuit through the sounder and its dry cell is closed by the relay (Fig. 77) and the sounder clicks. This is theFig. 77.Fig. 78.
usual arrangement in a simple telegraph office. The sounder in the first part of the above experiment can be replaced by an electric bell (Fig. 78) and the key by a push button, thus showing the arrangement of the ordinary doorbell.
Electricity can be developed by chemical action as well as by friction, and many different kinds of electrical cells have been invented. Themost simple of these is a sheet of copper and a sheet of zinc placed so that they do not touch and put in a dish containing dilute sulphuric acid (Fig. 79). The current developed by this cell is very weak. At the present time dry cells are used for almost all ordinary purposes in which electric batteries are needed.
The history of the development of our
Fig. 79.
knowledge of primary cells and current electricity is exceedingly interesting and important, but it cannot be dwelt upon here. In 1832 an American, Samuel F. B. Morse, invented the commercial telegraph. This was the first step in the wonderful progress that has been made during the last century in communicating rapidly between distant points. The necessary instruments used in this form of communication are a sounder (Fig. 80) and a key (Fig. 81). The sounder is simply anFig. 80.Fig. 81.
electro-magnet such as was made in Experiment 14, arranged to attract a piece of soft iron held at a short distance from it by a spring. When this piece of iron is attracted toward the magnet, it strikes on another piece of iron, making a click, and so remains drawn to the magnet as long as the circuit is kept closed. Thus long and short clicks can be made. Morse arranged a combination of these long and short clicks to represent the alphabet. Thus he was able to send words from one station to another. Experiment 87 illustrates how a simple telegraph can be arranged.
Many improvements have been made since Morse first sent a dispatch between Washington and Baltimore, but his dot-and-dash alphabet and the electro-magnet sounder and the key are still in use.
WIRELESS TELEGRAPH STATION, LOS ANGELES.
Since 1832, the land has been strung with telegraph wires and the ocean girdled with cables, and now an important event occurring in any part of the earth is known almost instantly in all other parts. The telephone, the wireless telegraph and the wireless telephone, all electrical devices, have added to the ease of communication so that the whole earth is brought into such close relation that every part knows what all the other parts are doing. No other form of energy which man has discovered is of such diversified usefulness as electricity.
82.Tornadoes and Waterspouts. -
Sometimes causes like those which produce a thunder-storm are so strongly developed that the indraft is exceedingly violent and a furious whirling motion is produced. Such storms are called tornadoes. The warm moist air rises rapidly and spreads out into a funnel-shaped cloud with the vertex hang- ing toward the earth. In the center of thewhirl the air pressure is much diminished
and the velocity of the inrushing whirling
A TORNADO.
Notice the funnel-shaped cloud.
wind is tremendous, being often sufficient to demolish all obstacles inits path.
The length of the path swept over by a tornado is rarely over thirty or forty miles and the width generally less than a quarter of a mile. The rate of progress in the Mississippi valley is from 20 to 50 miles an hour, usually in a northeasterly direction. These storms are often wrongly called cyclones. When storms of this kind occur at sea, a water column is formed in the funnel-shaped part of the storm and they then receive the name of waterspouts.
83.Cyclones. -In the belt of westerly winds are found, as has al-ready been noted, large storm areas called cyclones. As the barometric pressure in the center of these areas is lower than that of the surround- ing region, they are marked "Low" on the weather maps. Into these low-pressure areas the air from all directions is moving, but on ac- count of the deflection due to the rotation of the earth, the wind does not blow directly into them, but produces great whirls in which the air moves spirally inward and upward.