JoLY — On the Reading of Meteorological Instruments. 163 



galvanometer shows that the descending wire in the instrument 

 has reached the mercury level. The rheotome is provided with a 

 counter, on which the number of contacts is recorded. Reference 

 to a Table gives at sight the reading of the instrument. This 

 being recorded, the commutator hand is moved over the next con- 

 tact, i. e. the second devoted to the instrument in question. The 

 current through the rheotome has now been shunted at the instru- 

 ment ; the galvanometer for a moment returns to its zero position, 

 but is immediately deflected again by the readjustment of the 

 instrument into contact with wire iii. In the interval the 

 movable limb of the distant instrument has returned to its 

 starting-point. 



These operations being consecutive and of a purely mechanical 

 nature, it will be evident that a clockwork arrangement, to take 

 automatically the readings of the various instruments and record 

 them on divided paper, need not be very complicated or expensive. 

 It will suffice to indicate briefly the nature of such an arrange- 

 ment. 



A drum bearing divided paper is provided with a metallic stud 

 at one extremity. This stud moves over the disc of the commu- 

 tator, shown in Plate IV. as attached to wire i. Intermittent 

 motion is imparted to the drum through a suitable cam. The 

 intervals of rest are alternately long and short. That is, at a 

 reading contact it halts, say, for two minutes, the reading being 

 taken and recorded on the drum; it moves forward, resting for, 

 say, thirty seconds at the succeeding contact piece, while the 

 instrument which has just been read is being restored to its zero 

 position. Its next halt is again for two minutes, when another 

 instrument will be in circuit ; and so on, the paper on the drum 

 being divided into longitudinal divisions for the separate record of 

 each, succeeding instrument. 



Parallel to the axis of the drum, and attached axially to the 

 movable arm of the rheotome (on wire ii.), a spindle carrying a 

 screw thread ■ revolves. The screw may be a fac-simile or an 

 enlargement of those on the distant instruments. A carriage, 

 with small pricker attached, moves on the screw. Separate clock- 

 work may work the rheotome, but gearing with the commutator is 

 provided, so that, on the drum coming to rest over a contact, the 



