Page:Popular Science Monthly Volume 20.djvu/697

Rh house I happened to be, she put her little instrument to "messenger," sent the current by pulling clown the spring, and in two minutes a messenger was at the door waiting to convey my message to the telegraph-office. Time passes terribly quick, and my energy is gradually disappearing, so that I shall have to pass over some of the things I wanted to show you, such as the process of engraving by the electromagnetic system; how toys were moved, boats made to swim about, and birds to sing. At the recent Paris Exhibition, the varied applications of electricity were simply wonderful and amusing. I have no doubt we shall have a great many of them exhibited, shortly, at the Crystal Palace, where I hope some of you may be able to go before your return to school. I will now show you how this system of producing electro-magnetism can be utilized for telegraphic purposes. On the table are fixed two complete sets of telegraph-instruments, very close to each other, but the set on the right hand belongs to one station, and that on the left hand to another station. You may fancy that the instrument which I touch, on the left, is in London, and that which Mr. Cordeaux touches, on the right, is in Liverpool. For every current of electricity I send from London, I do a little work in Liverpool, and that results in motion, producing sound. The mere depression of my "key" produces electro-magnetism in Liverpool, which attracts a piece of iron and produces a sound. [Illustrated by working the instrument.] The battery attached to my key is in London, and the depression of my key sends on a current which arrives at Liverpool, produces magnetism and sound. By depressing the key rapidly or slowly the sounds may be made correspondingly short or long. [Illustrated.] By an arrangement of dots and dashes, the letters of the alphabet are represented, and experienced clerks can read off the sounds and translate them with astonishing rapidity. To illustrate this to you, I will ask your secretary to write down a short sentence, unknown to my assistants, which shall be sent by one of them, Mr. Cordeaux, on one instrument (supposed to be in Liverpool), and read off by Mr. Cooper on the other instrument (supposed to be in London). [The secretary then handed in a slip of paper to Mr. Preece, and the message it contained was read off by Mr. Cooper, "A merry Christmas to the juveniles."] That is the operation of telegraphy. The sounds are read off as clearly as ordinary spoken language, though mistakes are sometimes made. For instance, a party of young school-girls, out for an excursion once, wished to advise their schoolmistress of their safe arrival at a certain point, and sent the message, "Arrived all right"; but the schoolmistress was horror-stricken to read the message as delivered to her, which read "Arrived all tight." Telegraphically, the difference between the two messages is not great, for the letter R is represented by two dots and a dash, while T is represented by a dash. Another error in transmission was where a message, "Five fathoms and four feet is ample