Fig. 519.—Balance.
The Pendulum. A “simple” pendulum is impossible to make, for we cannot put the connecting line between the “bob” and the clock-work out of consideration, so “simple pendulums” are looked upon as “mathematical abstractions.” The most modern clocks have what is called a “deadbeat” escapement, and a compensating pendulum. Clocks are liable to alter by reason of the state of the air and varying temperature, and until all our clocks are placed in vacuo we must be content to have them lose or gain a little. There is a magnet arrangement by which the Greenwich Observatory clocks keep time by compensation, corresponding with the fall or rise of the syphon barometer attached to it. The description need not be added. We may here state that detailed descriptions of all the instruments used in the Observatory, together with full information as to their use, will be found in a very interesting work by Mr. Lockyer, entitled “Stargazing,” to which we are indebted for some corrections in our summary.
Fig. 520—Regulator.
We have spoken of solar time and sidereal time, and no doubt someone will inquire what is meant by mean time—an expression so constantly applied to the Greenwich clock time. Stellar time, we have seen, corresponds to the daily revolution of a star or stars. Solar time is regulated by the sun, and this is the astronomical time generally observed, except for sidereal investigations. But the sun is not always regular; the orbit of the earth causes this irregularity partly. The earth moves faster in winter than in summer, so the sun is sometimes a little fast and sometimes a little slow. Astronomers therefore strike an average, and calculate upon a Mean Sun, or uniform timekeeper. Mean time and true (apparent) time are at some periods the same—viz., on the 15th of April, on the 14th of June, on the 31st of August, and on the 24th of December. Twice it is after, and twice before it. The time occupied by this “mean” sun passing from the meridian and its return to it, is a mean solar day, and clocks and chronometers are adjusted by it.
Fig. 521.—Fusee and spring.
Twenty-four hours represents a complete revolution of the heavenly bodies. The mean solar time is 23h 56´ 4·091″, while twenty-four hours of mean time are equal to 24h 3´ 56·55″ of sidereal time. The difference between the times is given by Dr. Newcomb as follows, and is called the Equation of Time:—
| Differences between Mean and Apparent Time. | ||
|---|---|---|
| February 10th | True Sun | 15 minutes slow. |
| April 15th | ” | Correct. |
| May 14th | ” | 4 minutes fast. |
| June 14th | ” | Correct. |
| July 25th | ” | 6 minutes slow. |
| August 31st | ” | Correct. |
| November 2nd | ” | 16 minutes fast. |
| December 24th | ” | Correct. |
Measurement of Distances.