It is possible to connect any of these clocks telegraphically with an instrument in the observatory, known as a chronograph. It consists of a cylinder with a sheet of paper around it, on which rests a pen connected with the telegraphic instrument which follows the beats of the clock. The cylinder is turned slowly by clockwork, and the pen, carried slowly along by a screw, describes a spiral on the paper with jogs or teeth in it about a quarter of an inch apart, caused by the beats of the clock. In this way the astronomer secures a visible record of the beating of his clock, or rather of the movements of his telegraphic recorder. Thus, if he has another key on the same circuit with the clock, connected with his chronograph recorder, and should touch it between the beats of his clock, it would put in an extra jog or tooth on his record, and it will show, what he could not have told in any other way, in just what part of the second he touched this key, whether in the first or last part of the second, and precisely how far from either end—that is, he can determine fractions of a second with great nicety.
A Short Section from the Paper Band of the Chronograph Cylinder, showing Tracings of Pen connected with Clocks: 1, seconds of sidereal clock; 2, both sidereal and common clocks; 3-10, the tracings of the mean-time clock fall steadily behind the other; 11, sidereal only; 12, connected with observer's key. The extra teeth show when a star passed each of the five spider lines. At the extreme right is a "rattle," put in to show where the observation is on the cylinder.
As a matter of fact, he has such a key at the telescope which he uses to make his observations in taking time, so that when he wishes to record the precise instant in which anything takes place which he is viewing through his telescope he has but to press the key in his hand and an extra tooth will be put into the record which the clock is making, somewhere among the regular teeth put in by the beating of the clock. Later, when he takes out the sheet he can see just where the tooth came, and so at what instant it was. If, now, he knows exactly what the instant was according to the true time as given in his almanacs—that is, what his clock ought to have shown at that instant—he can tell how nearly right his clock is. Once knowing how this clock is, it is a simple calculation to find how the clock which sends the signals is running, and to alter it if needed in a manner we shall describe later.
The observations the astronomer makes use of to determine these instants of time are upon the "clock stars." He uses a rather small telescope, known as a transit. It is placed with the nicest accuracy in a north-and-south line. It can turn over vertically, but can not move sideways out of its line. Its alignment is kept perfect by occasionally sighting some small mark a few rods from the observatory, either north or south.
If the astronomer points this transit, say, halfway up the southern heavens and sees a star pass across the center of its field he knows that that instant gives, as it were, the "noon mark" of that star. If it is one of the "clock stars," he knows by his tables what that instant of time is—should be—by his clock.
We have seen what his means are of comparing his clock and his observations. But observe, now, how much pains he takes to get the most exact observations.
To begin with, he must have calculated to a nicety his location. The director of an observatory always knows where he is located in a sense that few other men do. The accuracy of a large part of his observations of any kind depends on his first having determined the latitude and longitude of his observatory within a very few feet. Then the data given by his tables are all modified, and adapted to conform to his locality.
There are stretched across in the eyepiece of his transit five spider lines. The central one is on the central line of the field of his instrument. In observing a star for time the astronomer watches it as it is carried by the rotation of the earth past each of these spider lines, and presses his key—that is, makes a record—as it crosses each line. Taking the average of these five observations, he makes the possible error very small. But, in addition to this, he also usually makes observations on at least four clock stars, which gives him twenty observations to average up and determine by. As he inspects the record of these observations which has gone upon the chronograph sheet along with the clock beats he is able to determine, after proper calculations, how his clock stands.
Such observations are made every three or four evenings, and thus the clocks are not given time to get far out of the way. It is not usual for a good clock to show a variation of more than half a second. If the astronomer finds that his clock which is sending the time is running a fraction of a second slow, he goes to it and lays on the top of the pendulum bob a minute clipping of metal, which is equivalent to shortening the pendulum an infinitesimal amount. When he takes his next observation he discovers how his clock has been affected, and again treats it accordingly. Thus the time that is sent out automatically by the clock is kept always correct within a small fraction of a second. Those who receive the time sometimes arrange electro-magnets near the pendulums of their clocks, which act with the beats of the observatory clock, and their attraction is enough to hold or accelerate the pendulums as needed to make them synchronize with the observatory clock.