144 Royal Astro7iomical Society, 



Royal showed by diagrams, is incorrect. If the battery and the re- 

 cording apparatus are near together, then it is only necessary to have 

 a single trunk-wire passing by all the stations (including the clock 

 as one) ; and to have a branch- wire passing through the recording 

 apparatus to the battery, and a communication from the other end 

 of the battery to the ground ; and also to have a branch-wire through 

 each station to the ground, interrupted in its ordinary state, but ad- 

 mitting of connexion at the will of the observer, or by the motion of 

 the clock. Then, at whatever place the observer completes the con- 

 nexion with the ground, the galvanic circuit through the recording 

 apparatus is completed. Or, in the most unfavourable case (which 

 is never likely to occur), of the battery at one extreme station, and 

 the recording apparatus at the other extreme station, it is only ne- 

 cessary to have one trunk- wire proceeding from the battery through 

 all the stations, and another trunk- wire parallel to the first proceeding 

 from the recording apparatus through all the stations; then, at 

 whatever station the connexion between these two trunk- wires (pro- 

 perly furnished with small branch- wires) is made by the observer, 

 the circuit through the recording apparatus is completed. 



The method of recording by the use of a fillet of paper, strictly 

 speaking, is used only by Dr. Locke. The equivalent method used 

 by Professor Mitchell is, to cause a circular disc to revolve with a 

 smooth and nearly uniform motion (by means of a Fraunhofer's 

 regulator), upon which disc the impressions of the style form a 

 dotted circle ; then, at the end of each revolution, a tooth upon the 

 axis of the disc takes hold of a fixed rack and moves the travelling 

 frame, which carries the centre of the disc, through a small space, 

 so that the traces of the succeeding circle are prevented from mixing 

 with those of the preceding circle. Professor Mitchell alludes to the 

 practicability of using a cylinder which turns upon a screw axis, so 

 that the traces will be made in a perpetual spiral ; and there can be 

 little doubt that this construction would be preferable to that of the 

 circular disc. Among other reasons, the habitual estimation, or the 

 measure, of the fractions of the seconds, would probably be more 

 accurate where the length corresponding to a second is uniform than 

 where it varies from one circle to another, as it does on the circular 

 disc. 



The momentary interruption of the circuit in Dr. Locke's method, 

 or the momentary completion in Professor Mitchell's method, for the 

 register of observations, is made by touching a key nearly similar to 

 that of a musical instrument. This key may be attached to the ob- 

 serving chair or to the astronomical instrument. Dr. Locke has not, 

 however, fully described the mechanism by which he makes his clock 

 interrupt the circuit. Professor Mitchell describes the action of his 

 clock in the following manner : — A delicate fibre attached to the 

 pendulum of the clock acts upon a " cruciform" lever [probably a 

 rectangular or "bell-crank" lever], and thus, in every double swing 

 of the pendulum, allows a metallic point to dip into a cup of quick- 

 silver, and to complete the galvanic circuit. Thus, in Professor 

 Mitchell's register, the clock-dots are made at every two seconds. 



