5 22 THE POPULAR SCIENCE MONTHLY. 



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 ^f our 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 gal- 

 vanic battery is so arranged that every swing of the second's pendu- 

 lum 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 an- 

 other 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 disturb- 

 ing 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 con- 

 tinue for many successive days and nights, he has to fall back on his 



