Dreyer — On Astronomical Transit Observations. 489 



A comparison "between this estimated moment and tlie true distance of 

 the transit from the preceding second-beat gives then the personal 

 error. This instrument has only been used a little by the inventor 

 himself, and it is too complicated. 



Simpler, and therefore depending less on the exact manufacture of 

 all the details, is the instrument used by MM. Plantamour andHirsch, 

 at the determinations of longitude in Switzerland.'^^' Like the in- 

 struments of C. Wolf and Kaiser, it makes use of electricity ; but in 

 another way, in conjunction with the " chronoscope " for measuring 

 very small intervals of time. This chronoscope is a fine clockwork 

 with two hands, which turn once, respectively, in 0^"1 and in 10^; as 

 the dials are divided into 100 parts each, the one can show O^'OOl, the 

 other one, 0'"1. The axis of the former hand — which moves the axis 

 of the latter by a toothed wheel — can be pushed backwards and for- 

 wards, by the establishing and interrupting of an electric current. In 

 the former case, a cog-wheel on the axis is pressed against another 

 wheel, which is moved by the clock and has 100 teeth, so that the 

 two wheels will move together after less than 0^-001. When the cur- 

 rent is closed again, the axis goes forward and the wheels separate ; 

 accordingly, the two hands are stopped. The passage of a In mi nous 

 point behind a wii'e suspends the current, while the observer himself, 

 in the moment he remarks the transit, closes it again. The hands of 

 the chronoscope will, therefore, indicate the personal error of the ob- 

 server, but, of course, only if it is negative ; as the hands in the oppo- 

 site case (when the observer closes the ctu-rent before it has been, 

 opened) are not moved at all, so that it can only be seen that the ob- 

 server has anticipated the transit, but not how much.f The artificial 

 star is produced by a board, movable by a pendulum, with a small hole 

 in it, through which the light of a gas-flame shines. Once during 

 each oscillation, in the moment it passes the vertical line, the pen- 

 dulum interrupts a metallic contact, and suspends the electric current, 

 thereby letting the hands of the chronoscope join in the motion of the 

 clockwork, until the observer, with a key, closes the current again, 

 and stops the hands. The pendulum, board, and gas-flame were 

 placed in the meridian-mark-room of the Observatory in jSTeufchatel. 

 An assistant has to move the pendulum towards the east ; the ob- 

 server lets it pass the vertical line towards the west, and presses the 

 key when he sees the star go back again and (when the pendulum 

 again is vertical) pass behind the movable wire in the transit instru- 

 ment, which he, before the beginning of the observation, has made 

 bisect the star. The metallic contact can be regulated by a micro- 



* Determination Telegraphique de la Diif erence de Longitude entre les Observa- 

 toires de Geneve et de Neufchatel. Par E. Plantamour et A. Hirsch, Geneve, 1864. 



t This case happened several times during Plantamour' s observations. Before 

 calculating the probable value for the personal error,^ he, therefore, left out an 

 €qual n\unber of the largest negative eiTors. 



