—————— lS 
ON COMPARISON OF MAGNETIC INSTRUMENTS. 95 
Comparison between Dip Circle Dover No, 94 and the Instrument 
in use at the Stonyhurst Observatory. 
Date Time Instrument Dip =-S=, 
H. M. : d 
August 18 . - 16 25 = 2 9-9 
7 <i 16 27 Ss 6 z 
August 19. . 16 1 = 21 \ ee 
cs oiwig 16 4 Ss 4:3 aie 
August 20 . : 15 23 = 1:2) _ 948 
. im < 15 22 Ss 40 | “ 
August 22 . : 11 20 s | 6-0) 3-2 
_ i ae 12 16 = | 2:8 f Fifer 
Mean . - : — = — —2'8 
In the account of the observations at Stonyhurst the Observatory 
instruments are indicated by the letter =. The measurements with 
the survey instruments were made by Mr. Watson; those with the 
Observatory instruments by Mr. Ronchette, who usually makes the regular 
observations. 
The dip observations were not very satisfactory. The needles used 
at the Observatory were not in good agreement, and after some trials it 
was determined to use the needles employed with No. 94 in the experi- 
ments with both instruments. Whereas, therefore, the dip observations 
elsewhere included any difference between the ‘bias’ of the needles 
ordinarily used at the Observatory and of dip circle 94, at Stonyhurst this 
element of error was eliminated, and the comparison was between the dip 
circles only. 
In this as in other cases each dip recorded in the table is the mean of 
the results obtained with the two needles. 
Valentia Observatory (Caherciveen). 
The observations at Caherciveen were made by Mr. Watson, using the 
survey instruments, and by the superintendent, Mr. J. E. Cullum, who 
used the observatory instruments. 
Since there are no self-recording instruments at Caherciveen, the 
observations were made in a slightly different manner from that adopted at 
the other observatories. A tripod was erected at a distance of about 
ten yards from the magnet house, and on the line joining the pillar and 
the fixed mark. Observations were then made simultaneously with one 
instrument in the magnet house and the other on the tripod outside. The 
instruments were then interchanged and the observations repeated. By this 
means it was possible to eliminate the effect of any change in the element and 
any difference between the value at the two positions. In the case of the 
dip observations only one set were performed in the above manner ; the 
rest were taken as at the other observatories. The lens of the vibration 
magnet was found to be loose, as it had not been screwed ‘ home’ after the 
lens and scale were interchanged when the observatory was moved from 
Valentia to Caherciveen. It was therefore screwed in as far as it would 
go, and the moment of inertia was again determined by Mr. Cullum. 
