Page:Popular Science Monthly Volume 29.djvu/538

522 place in the heavens where he knows the star is to cross. He has his clock or chronometer by his side ticking seconds or half-seconds. A little lamp sends a ray into the tube of the telescope, so that he can see the spider-lines. With paper and pencil in hand he stations himself in front of the tube. The star enters the field of view and moves toward the first spider-line. He glances at the clock, catches the time by the second's hand, and counts the ticks. Three—four—five—six—the star has just crossed a line. Estimating the tenths of a second, he records the time on the paper. All this while he is noting the beats of the clock, and, when the star reaches the second-line, he is ready to record another transit, and so on through. The mean of all these times is the time of crossing the central line by the sidereal clock. But in the "Nautical Almanac" this time is given accurately, and a comparison of the two shows his clock error.

Instead of recording the transits by the "eye-and-ear" method above described, there is an easier way by simply tapping the key of an electric circuit at the time of transit. This makes a record on a "chronograph," which can be read at leisure.

A chronograph consists of a brass cylinder, on which is fastened a sheet of paper. This is placed with its axis horizontal, and is revolved uniformly by clock-work. A pen rests with its point against the paper, making a mark around it. By a slight longitudinal motion this mark does not come around into itself, but advances a trifle, being like the thread of a screw, running from end to end. A current from a galvanic battery is so arranged that every swing of the second's pendulum causes an electro-magnet to attract the armature to which the pen is attached, and makes a break in the mark. Hence there is a series of breaks separated by intervals of a second. When the observer notes a transit, he, by his key, makes galvanic connection and interjects another break in the line. The position of this break among the seconds tells when the transit occurred, the fractions of a second being readily read.

He thus knows sidereal time; a little reduction gives him the mean solar time of the place of observation, from which the time at any other place whose longitude is known is directly deduced.

His telescope, to avoid all possibility of error, must be in perfect adjustment. The axis must be level; it must point east and west; his spider-line must be correctly placed in the tube; the pivots of the axis must be of equal size and uniformly round, and the axis must not bend under the weight of the tube. All these sources of error are carefully guarded against, but, as human powers are finite and disturbing causes very plentiful, errors will be introduced in various directions. So he seeks to nullify these by taking many stars in different parts of the sky, and from the varying errors he deduces what part belongs to the clock and what to the instrument. Should cloudy weather continue for many successive days and nights, he has to fall back on his