60 



SCIENCE. 



[Vol. II., No. 23. 



are used, and yield satisfactory results. The record 

 is made on paper sensitized by the bromo-iodide 

 process. The paper is sensitized at the observatory. 

 Each trace contains two days' record; and the record 

 is absolutely complete and continuous, except the 

 time lost in changing papers to begin a new record, 

 and iu ' moving spots,' or shifting the luminous dots 

 to get the second day's record on the same sheet. 

 The time required for the first operation is from 

 seven to eight minutes ; for the second, from two to 

 three minutes. Thus only about ten minutes are lost 

 in two days, or an average of five minutes per day, — 



,ra« '"■»• 



a ' ' ' S To = 7S 20 



a quantity too small to be of any importance on any 

 occasion thus far observed. 



One minute of time on the traces is represented 

 ty Tou of an inch approximately, and a movement 

 through one minute of arc by the unifilar magnet 

 is represented on the trace by tuu of an inch. A 

 motion of the bifilar magnet of one scale division, 

 represented on the trace by 0.02Y inch, corresponds 

 to a change of horizontal force of about its g^g part. 

 The traces can readily be read off within half a scale 

 division, or changes of force of its Tromr part are 

 recorded. This adjustment has not proved too sen- 

 sitive, as the luminous dot has never left the recording 

 cylinder, except once for a short time during the great 

 magnetic storm of November, 1882. 



Visitors are admitted to the observatory, and the 

 traces generally show their presence by a break in 

 the curve. 



The instrument records, as is well known, changes 

 of declination, changes of horizontal force, and 

 changes of vertical force. Each of these changes 

 is recorded on a separate sheet, or trace as it is 

 called ; and thus, on an average, forly-five traces 

 are produced each month. Tliese traces are six 

 inches by sixteen inches and a half, and are made 

 on plain photographic paper prepared for use at the 

 observatory. 



This preparation consists of two processes, 

 salting and silvering. The salting process, as 

 it is called, consists in soaking the paper from 

 ten to fifteen minutes in a bath of iodide and 

 bromide of potassium, with a little tincture of 

 iodine added, after which the paper is hung 

 up to dry. This process is carried on in the 

 daylight. 



The silvering or sensitizing process is car- 

 ried on in a room as dark as can well be made, 

 and then lighted up dimly with a red lantern. 

 Some difficulty has been found in keeping the 

 room dark enough, and on some occasions 

 the silvering has been done at night. 



For silvering, four wooden trays are placed 

 in a row: the first containing a bath of nitrate 

 of silver, acetic acid, and water; the second, 

 distilled water; the third, a weak solution of 

 chloride of ammonium; and the fourth, dis- 

 tilled water. A sheet of salted paper is then 

 floated on tray no. 1, special care and some 

 skill being required to prevent (a) any of the 

 solution from getting on the back of the sheet, 

 and (6) any air-bubbles from clinging to the 

 front side of the sheet. The first defect pro- 

 duces stains, and the second, spots. In about 

 nine minutes the paper is transferred to tray 

 no. 2, being floated on as in the case of no. 

 1, and a new sheet is floated on tray no. 1. 

 In about nine minutes more, the sheets are 

 moved forward, as before; the paper in no. 2 

 is floated on no. 3; that in no. 1 is transferred, 

 as before, to no. 2, and a new sheet floated on 

 no. 1. This continues till tray 4 is reached; 

 after which the sensitizing is complete, and 

 the paper is then hung up to dry in the dark. 

 Special care is necessary in hanging up the wet 

 paper to avoid stains from the fingers, from the 

 line, or from the pin which holds the paper on 

 the line. 



After drying thoroughly, the papers are taken down, 

 packed in a large envelope, and kept in a dark drawer 

 to be used as needed. From this envelope the sheets 

 are transferred to the three recording cylinders pre- 

 pared to carry them. They remain two days upon 

 the cylinders, and thus receive two days' record. At 

 quarter-past nine a.m. of each alternate day the 

 papers are changed. 



Over the central driving-clock is hung a heavy 

 orange-flannel curtain. To change papers, the at- 

 tendant, with the envelope of sensitive paper, goes 



