706 SEC. 14. METEOROLOGY. 



The apparatus consists therefore essentially of two parts, one in which the 

 prepared paper can be exposed for definite periods of time to the action of 

 the light, and a second part in which the intensity of the tint obtained can 

 be determined. 



The paper (A), previously salted by immersion for five minutes in a 3 per 

 cent, solution of sodium chloride, and cut into .strips, is silvered by floating 

 for two minutes in a 12 per cent, solution of silver nitrate (B), contained in 

 the loi% trough (C), and afterwards dried on the reel (D). 



The prepared paper may be preserved either before or after exposure in 

 the dark box (E). It is next wound on to the bobbin of the insulator (F), 

 which is placed in electric communication with the clock (H) by means of 

 the battery (K) ; the free end of the paper, passing over the large wheel, 

 being held in position by means of a small pin on the inside circumference of 

 the wheel. 



When a current of electricity passes, the magnet attracts the armature, and 

 the wheel moves through a small space ; the circuit is immediately broken 

 and the armature released, and this slight movement of the wheel is repeated 

 every time a current of electricity passes through the apparatus. The insula- 

 tor is provided with a cover (G), in the top of which is a small circular hole, 

 against which the prepared paper is pressed by means of a spring, and as the 

 Avheel revolves, fresh portions of the paper are successively brought under 

 this hole, and thus exposed to the action of the light. The mechanism of the 

 clock is so arranged that discs of prepared paper shall be exposed to the 

 action of the light each hour for 10 different periods of time, which have 

 been exactly determined, varying from 2 to 30 seconds, the object of this 

 being to obtain, either with the feeble light of the morning, or the strong 

 light of mid-day, a tint neither too light nor too dark to be read off. This is 

 accomplished by means of a large metal disc in the clock, which revolves 

 once in two minutes, and is in metallic connexion with one pole of the 

 battery. On the face of the disc are placed 11 platinum pegs, arranged at 

 equal distances from the centre of the disc, but at such different distances 

 from one another that the first 10 intervals correspond as closely as possible 

 to 2, 3, 4, 5, 7, 10, 12, 17, 20, 30 seconds respectively (their values being 

 afterwards experimentally determined to one-fifth of a second), whilst the 

 value of the last interval is of no importance. The other pole of the battery 

 is connected with a metallic lever tipped with platinum, the insulator forming 

 a part of the circuit. 



Each hour this lever is lowered mechanically so that it comes in contact 

 with the first platinum peg ; the circuit is completed, the magnet in the insu- 

 lator attracts the armature, causing the wheel to make a small fraction of a 

 revolution, and a fresh surface of paper is exposed to the light. Contact is 

 immediately broken, the peg passing away from under the lever, to be again 

 made when the next peg passes by, and so on through one revolution of the 

 disc. After the 1 1th peg has passed by, the lever is automatical!} 7 raised, and 

 remains out of contact till the next hour. The last exposure, therefore, cor- 

 responds to the interval, the value of which has not been determined, this 

 portion of the paper remaining exposed until the next hour. 



After the apparatus has been in work for 24 hours the strip of paper is 

 exhausted, and must be replaced by a new one. 



On the paper thus exposed will be found repetitions of a scries of 10 discs, 

 of a tint which in each series gradually increases in intensity, separated by 

 one black disc when the paper has been exposed for the whole hour. One 

 half of two or three of the discs in each series is cut away by the semi- 

 circular punch (L), and the intensity of the tint in each case read off. For 

 this purpose the apparatus under the glass case (M) is employed. Bound 

 the drum (cr) is pasted the graduated strip (6), the intensity of the tint on 



