This section is from "Scientific American Supplement Volumes 275, 286, 288, 299, 303, 312, 315, 324, 344 and 358". Also available from Amazon: Scientific American Reference Book.
AT 9 A.M. on Wednesday, September 13, the correspondent of a press agency dispatched a telegram to London with the intimation that the great battle at Tel-el-Kebir was practically over. It may possibly astonish not a few of our readers (says a writer in the Echo), to learn that this message reached the metropolis between 7 and 8 o'clock on the same morning; and, in fact, had an unbroken telegraphic wire extended from Kassassin to London, Sir Garnet Wolseley's great victory might have been known here at 6:52 A.M., or (seemingly) at a time when the fight was raging and our success far from complete. Nay, had the telegram been flashed straight to Washington in the United States, it would have reached there something like 1 h. 44 m. after the local midnight of September 12. Paradoxical as this sounds the explanation of it is of the most simple possible character. The rate at which electricity travels has been very variously estimated. Fizeau asserted that its velocity in copper wire was 111,780 miles a second; Walker that it only travels 18,400 miles through that medium during the same interval; while the experiments made in the United States during the determination of the longitudes of various stations there still further reduced the rate of motion to some 16,000 miles a second. Whichever of these values we adopt, however, we may take it for our present purpose, that the transmission of a message by the electric telegraph is practically instantaneous. But be it here noted, there is no such a thing as a hora mundi or common time for the whole world. What is familiarly known as longitude is really the difference in time, east or west, from a line passing through the north and south poles of the earth; and the middle of the great transit circle is the Royal Observatory at Greenwich. If in the latitude of London (51° 30' N.), we proceed 10 miles and 1,383 yards either in an easterly or westerly direction, we find that the local time is respectively either one minute faster or one minute slower than it was at our initial point. Let us try to understand the reason of this. If we fix a tube rigidly at any station on the earth's surface, pointing to that part of the sky in which any bright star is situated when such star is due south (or, as it is technically called, "on the meridian"), and note by a good clock the hour, minute, and second at which it crosses a wire stretched vertically across the tube, then after a lapse of 23 h. 56 m. 4.09 s., will that star be again threaded on the wire. If the earth were stationary--or, rather, if she had no motion but that round her axis--this would be the length of our day. But, as is well known, she is revolving round the sun from left to right; and, as a necessary consequence, the sun seems to be revolving round her from right to left; so that if we suppose the sun and our star to be both on the wire together to-day, to-morrow the sun will appear to have traveled to the left of the star in the sky; and the earth will have that piece more to turn upon her axis before our tube comes up with him again. This apparent motion of the sun in the sky is not an equable one. Sometimes it is faster, sometimes slower; sometimes more slanting, sometimes more horizontal. Thus it comes to pass that solar days, or the intervals elapsing between one return of the sun to the meridian and another, are by no means equal. So a mean of their lengths is taken by adding them up for a year, and dividing by 365; and the quantity to be divided to or subtracted from the instant of "apparent noon" (when the sun dial shows 12 o'clock), is set down in the almanac under the heading of "The Equation of Time." We may, however, here conceive that it is noon everywhere in the northern hemisphere when the sun is due south. Now the earth turns on her axis from west to east, and occupies 24 h. in doing so. As all circles are conceived to be divided into 360°, it is obvious that in one hour 15° must pass beneath the sun or a star; 30° in two hours, and so on. The longitude of Kassassin is, roughly speaking, 32° east, so that when the sun is due south there, or it is noon, the earth must go on turning for two hours and eight minutes before Greenwich comes under the sun, or it is noon there, which is only another way of saying that at noon at Kassassin it is 9 h. 52 m. A.M. at Greenwich. It is this purely local character of time which gives rise to the seeming paradox of our being able to receive news of an event before (by our clocks) it has happened at all.