applied to Terrestrial Magnetism. 457 
mencing with the arc of 20°, and noting the time of completion of every 10th vibra- 
tion during the interval by a chronometer. The needles are short cylinders, 24 
inches long, and .13 of an inch in diameter; such being the form and size recom- 
mended by the same skilful and indefatigable observer. 
lace Date Cyl. | No. | ‘Lime Intensity 
London | July 8-19 | R(c)| 19 | 441."59 ; 
Aug. 30,31 6 | 441.46 
Mean 25 441.53 1.0000 
454.06 9456 
438.57 
* Dublin | Aug. 16 — | 38 
) s 
6 4:39.07 
14 
3 
London | July 19,20| R (d) 
Aug. 28,29 | —— 
Mean —— 438.82 1.0000 
Dublin | Aug. 14. | —— 452.11 9421 
London | Sept.19-22; L(1)| 3 | 235.98 1.0000 
Dublin 12-15) —— | 4] 243.52 .9390 
CII) | London | Sept.19-22) L(2)| 3 | 284.01 1.0000 
Dublin 12-15, —— | 3] 293.31 .9376 
London | Oct. 23,24} L(1)| 3 | 235.42 1.0000 
London | Oct. 23,24) L(2)| 4 | 283.11 1.0000 
Dublin | Nov. 5,6 | ——| 3] 293.27 .9319 
The mean of these three determinations gives the horizontal component of the 
magnetic force in Dublin equal to .9380, that in London being unity. To deduce 
from this the ratio of the total force at the two places, we must know the amount 
of the dip at each. The following are the results of the observations made with the 
needles (1) and (IV), in the autumn of 1835. In taking the mean, only half the 
weight has been allowed to each observation with the latter,—the number of readings 
from which the dip is deduced in the statical method being half of that taken in the 
ordinary process. 
* Place | Date Needle No. Dip 
Dublin Sept. 4-15 I 6 70° 53.5 
IV 6 70 52.0 
Mean 12 70 53.0 
London Sept. 19-25 I 9 69 6.3 
Sept.—Oct. IV a 69 9.8 
Mean 16 HOF 725 
* The observations of comparison (I) were made by Captain James Ross. 
