The Chronograph.— There are two prin cipal methods by which the astronomer notes the time of occurrence of an instantaneous phenomenon to a fraction of a second. On the older system, which is not without its ad vantages, the observer, looking at his dock, counts its beats until the occurrence of the phenomenon he is to observe. We may take, as an example, the occultation of a star by the moon. He sees the limb of the moon ap proaching the star until it is clear that, in a few seconds, it is going to pass over it and hide it from view. Then looking at the clock, he listens to the seconds, mentally counting the yet hidden, while before the next beat the star has disappeared from view. He estimates how many tenths of the interval between the beats of the clock had elapsed when the star dis appeared, and records the hours, minutes, seconds and tenths in his note-book. The skilled observer will seldom be more than a few tenths of a second in error in this estimate. It requires long practice, and much natural aptitude, to be able to make an accurate obser vation in this way. The method has also the inconvenience that there is no permanent record except that which is written down at the moment, so that, if the observer has made an error of any kind, he has no direct way of detecting it except by subsequently discover ing that something must be wrong. This diffi tulty is avoided by means of a chronograph. In the form commonly used, the chronograph consists essentially of a cylinder, genera ly about eight inches in diameter and one or two feet in length, revolving on its axis by clock work at the rate of one turn a minute. Around the cylinder is stretched a sheet of paper, which is carried with it in its motion. The sheet is pressed by a pen, pencil or other point, so as to leave a mark on the paper as the cylinder revolves. The pen is carried by a little carriage moving slowly forward from one end of the cylinder to the other at a rate of about one-tenth of an inch, or a little more, in a minute. Consequently, the point describes a spiral line on the paper as the chronograph goes through its successive revolutions, until the pen arrives at the farther end of the cyl inder. This may take a period of two, three or four hours, according to the adjustments. The pen is connected with an electro-magnet, the current around which passes through the works of the clock. The arrangement is such that at every beat of the clock, or sometimes at every alternate beat, the electric current is either closed or broken. With each closing or breaking of the current a slight motion is given to' the pen so that the seconds are marked on the paper on the revolving cylinder. The same or another current also passes through a key held in the hand of the observer. When the latter sees the moment of the phe nomenon he is to note approaching, he holds the key in his hand, and presses it at the exact moment to be recorded. A motion is thus given to the pen, and the position of the signal on the paper among the signals given by the clock shows the moment to a fraction of a second at which the signal was given.
Different systems are used based on this general principle. There are various ways in which the pen marks the clock beats on the paper. In that mostly used in this country, the pen is not raised from the paper, but is given a sudden lateral jerk, producing a notch in the line, as shown in Fig. 6, which is a copy of a small portion of a- chronograph record. On another system the pen simply makes dots on the paper at each beat of the clock. Some times the current passes around the electric magnet all the time except at the instant a signal is made. Then one and the same elec tric circuit is used for both the clock and the observer. Sometimes the clock only makes the circuit at the moment of its beat; then the circuit at the command of the observer is a second one, which he makes by pressing the key. The main point in all systems is that the beginnings of the minutes all come under each other so that, by taking the sheet off of the used for two distinct purposes. One is the de
termination of the right ascensions of the heavenly bodies; the other the determination of their declinations. It will conduce to clear cylinder, and spreading it out, writing in the minutes and the lines of seconds, the ob server can determine the exact moments at which every one of any number of signals were made while the chronograph was running. For example, in Fig. 6 it will be seen that the ob server pressed the key at 12 m. 3.4 s. and again at 13 m. 2.4 s.
The Spirit Level.— Another appliance much used in astronomy is the spirit level. It serves to set the axis of an instrument exactly horizontal. It consists of a glass tube, ger. erally six or eight inches long, of which the rounded surface is not a perfect cylinder, but is formed by the revolution of the arc of a very large circle around its chord. The tube is therefore of the shape shown in Fig. 7, slightly larger in the middle part than at the two ends. The amount of bulging is, however, so slight that the eye cannot perceive it. In the most delicate levels, a section of the curved surface is an arc of a circle perhaps half a mile, more or less, in diameter. The tube is nearly filled with chloroform or ether. Water, or even alcohol, is not liquid enough for the purpose. A small vacant bubble is left at the top of the cylinder, as shown at A B in Fig. 7. When this bubble is in the middle of the tube, the axis of the level is perfectly horizontal. The remainder of the level is sketched in Fig. 8, which shows the level completely mounted, so that it can be set on the horizontal pivots of the instrument of observation. The true horizontality of the pivots is tested by reversing the level end for end, reading the position of the bubble at each setting. Details need not be entered into at present, as we only wish to make the principle of the instrument clear. Nearly all instruments for astronomical measurement are made by putting together some combination of the devices we have described. The two combinations most used in astro nomical observations are the Meridian Circle or Transit Circle, which are the same in principle, and the Equatorial Telescope.
The Meridian Circle.— This instrument is ness to consider these two functions separately and begin with the instrument as adapted to the first purpose. In this form it is called the transit instrument and is shown in Fig. 9. It consists essentially of a telescope mounted on a horizontal east and west axis P Q, the hori zontality of which is tested from time to time by a spirit level. As thus mounted it will be seen that the telescope cannot move out of the meridian; by turning it on its axis, its line of sight marks out the meridian. Consequently, if an observer looking into it sees a star, or other heavenly body, he knows that the star must be near the meridian. To make the ob servation more exact, a system of spider lines, shown in Fig. 14, is stretched across the focal plane, as already described. The middle line is so adjusted as to mark the meridian with the greatest possible exactness. The result is that the observer, looking into the instrument, sees these spider lines, and he may also see a star moving toward the meridian by virtue of its apparent diurnal motion as shown in the figure, where it is about to cross the meridian line. Watching it with a key connected with the electric circuit of the chronograph in his hand, he taps the key at the moment the image of the star crosses each of the lines. The middle line marks the passage across the meridian. The other lines are used in order to secure greater exactness by taking the mean of all the transits across the separate lines. Thus, by pointing his instrument into any part of the meridian, the observer may determine the times by his sidereal clock at which any number of stars crossed the meridian of his place.