88 REPORT—1896. 
will be able to ensure the maintenance of the magnetic observations 
under the best conditions. 
The instruments used in the comparisons which have been carried out: 
are magnetometer No. 70, by Messrs. Elliott Brothers, and dip circle 
No. 94, by Dover. They had been used in the recent magnetic survey 
of the United Kingdom, and are indicated hereafter, as in the published 
account of that survey, by the letter S. 
They were in the first place compared again with the Kew standards 
at Kew by Professor Riicker. Some changes had been made in No. 70, 
so that the differences with Kew are not strictly comparable with those 
which have previously been published. In the case of the dip circle the 
results were in satisfactory accord with earlier measurements. 
The method of comparison of the instruments was altered, so as to 
diminish the risk of error from variation in the zeros of the self-recording 
instruments. To this end alternate sets of observations were made with 
the instruments to be compared at short intervals on the same day, so 
that the zero lines of the self-recording instruments, by which these 
observations were reduced to the same time, could not have appreciably 
altered (see ‘ Phil. Trans.,’ 1896, 188, p. 11). 
After the instruments had been compared with those used in the 
other observatories they were brought back to Kew, and a similar set of 
comparisons to that above described Were made, chiefly by Mr. Watson. 
The results of the two sets of experiments are entered in the following 
tables. 
As it was desirable to show that the Kew standard instruments 
gave normal results when used hy Professor Riicker and Mr. Watson, 
observations were made with them by the Observatory officials on days 
near those on which the comparisons were carried out, and the readings 
for the zero lines of the self-registering instruments were determined so 
as to serve as a check on the corresponding values obtained when the 
survey instruments were compared with the standards. These observa- 
tions are indicated by an asterisk. 
The most convenient way of making the comparison is as follows :— 
Let Cy) and C be the readings of the self-registering instruments at: 
the time when the value of the element was determined by the Kew 
standard (K) and No. 70 (S’) respectively. Then K—C,)=Z,) and 
8’—C=Z are the values of the zero line of the self-registering instrument 
according to the two observations. But if the observation with No. 70 
had been made at the same instant as that with the Kew standard, and if 
the zero line remained unaltered in the interval which actually occurred 
between the two experiments, the simultaneous values of the element 
given by the two instruments would have been K and S8=8’+C,)—C. 
“. K—S=K—C,—(8’—C)=Z)—Z. 
On October 12 the self-registering instruments at Kew were disturbed, 
owing to some work which was being done in the room in which they are 
placed. 
The Astronomer Royal was therefore good enough to supply us with 
the values of the elements given by the Greenwich instruments at the 
time of our observations at Kew, and by using these instead of Cy and C 
the proper allowance for diurnal variation and disturbance could be 
made. 
In the following tables the number of whole degrees in the values of 
Z, ana Z is omitted :— 
