number, the time of one oscillation is obtained. Of course, the clock or chronometer which furnished this time might be running too fast or too slow. However, it has been customary to determine the rate of the timepiece by making an astronomic observation before swinging the pendulum and then again after. This would give the amount of time gained or lost in the interim, but does not prove that this amount, or even more, was not the change within the interval of swinging, or that the keeping of time was not perfect while the pendulum was swung, and the error occurred either before or after.
From this uncertainty arose the need to eliminate time error, and it has been most ingeniously met in the survey pendulum. Here two pendulums are employed—one at one station and one at another, connected by a telegraph wire. Each is made to record its own coincidence with a beat of one and the same chronometer, so that if the chronometer has a constant rate for a minute or two it is sufficient. The chronometer at the other station is then used to eliminate such errors as might arise in the transmission of signals. In this way the relative time of the oscillations of two pendulums is known with absolute accuracy, and from these relative times relative gravity is obtained, and from relative gravity we have relative distances to the earth's center, or the shape of the earth.
In this enlightened age it is not necessary to enumerate reasons why we should know the shape of the earth. It enters as a potent factor in astronomic computations; it is indispensable in map-making, and no boundary line can be drawn without its aid. Besides carrying on the special cartographic functions prescribed for it by law, the United States Coast and Geodetic Survey has lost no opportunity to improve our knowledge of this important factor, and under no régime has the survey so completely filled this dual purpose as under the superintendency of Prof. Mendenhall.
We do not measure the earth with a span, but with a pendulum one span in length we find its shape.
The publication of an Index to the Names and Authorities of all Known Flowering Plants and their Countries, which was contemplated by Mr. Darwin, has been undertaken at the Clarendon Press, under the direction and supervision of Sir J. D, Hooker. Part I of the work is now ready, and Part II is well advanced.