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It will now and again happen to the seeker after knowledge that he will have to unlearn as well as to learn ; but it will be a rare experience for him to have to call in question such a supposedly fundamental truth as that of the invariability of the earth s latitude. If there is one fragment more than another of our childhood s geography lesson that abides ever with ut , it is this: that the earth turns upon its axis. And now we are told that it does not, and that, as a consequence, it is literally true that the parallels of latitude are perpetually shifting—not much, it is true; but sufficiently to make it comically possible, as was once suggested, that certain dwellers in the proximity of the Canadian border line never know for more than six months together in which country they live. The axis of the earth, or, to speak more accurately, the axis of the earth s figure, is an imaginary line. passing through the center of the earth, and term i-nating at its two flattest points, known as the North and South Poles. Up to the year 1888, it was supposed that the earth rotated about this axis. If this had been true, the latitude of any given spot, as de-termined by observation, should have been invariable. As a matter of fact, it had been noticed, even as far back as the last century, that there was a slight, but perceptible, variation. The latitude of a given spot, as shown by two observations taken at different times, would be found to vary. Between the years 1884 and 1888, Dr. S. C. Chandler gathered together all the observations that had from time to time been made, . a careful analysis, was able to prove that th ese variations are accounted for by the fact that the earth does not rotate about its axis of figure, as above described, but a bout another axis, which he ca lied the axis of rotation. This axis of rotation bisects the axis of figure at its center, and always preserves the same direction in space; b ut its poles I ow l y d escribe . a circle about the poles of the axis of figure. From this consideration it is evident that the parallels of latitude do not preserve the same planes relative to space ; but have an oscillatory motion. Hence the variation. The motion is fairly well illustrated by a spinning top, whose center of gravity remains in the saine vertical line, while the peg and the head describe two circles about this vertical line. The motion of any parallel lines on the top will roughly approximate to the motion of the lines of parallels of latitude on the earth s surface. The above illustration will only ap-proximately show this motion of the earth, for the reason that th e 1 atter IS complex, being made up of two superposed motions. The pole of rotation moves in a small circle which is itself moving around the pole of the earth s figure. The period of the smaller circle is between 423 and 434 days; that of the larger between 361 and 36972 days. The radius of the smaller circle is 14 feet. The center of the circle itself travels in an ellipse, the major axis of which is about 25 feet, and the minor about 8 feet. A remarkable verification of Dr. Chandler s discovery was afforded by a .series of tidal observations extending over 35 years, two of which were taken on the Pacific Coast and one on the Atlantic. These show a mean time of oscillation of the sea s level of 431 plus or minus 4 days, which agrees remarkably with the period of revolution as mentioned above. Newcomb had pointed out that if the theory of the revolution of the axis of rotation were true, low tide at any spot should occur when the pole of rotation lay nearest that spot—a suggestion with which the above tidal observations fully agree.
