70 
THE EEY. STEPHEN J. PERRY ON MAGNETIC OBSERVATIONS 
Each of these values is the mean of the monthly determinations obtained by observa- 
tions of deflections at DO and 1/3 feet. 
For the dip three needles have been employed, Nos. 1 and 2 from 1863 to August 
1868, and Nos. 1 and 3 since the latter date. A correction has been applied since 
August 1868 for the change of needle; this correction = — O' 20" for the mean value. 
The observations have always been taken by the Director of the Observatory, viz. by 
the Rev. W. Sidgkreaves, from 1863 to September 1868, and since that date by myself. 
Unfortunately the station of observation has not been always the same, nor perfectly 
free from disturbing influences. The first station was a stone pillar erected in 1858 in 
the botanical garden of the college. In June 1864 the N.W. wall of the garden was 
replaced by an iron railing, whose nearest distance from the magnetic pillar is 91 feet. 
In January of the following year a three-quarter inch iron pipe was laid down on the 
S.E. side of the pillar, at a distance of 38 feet 9 inches; and in September 1867 a 
second iron pipe, 1J inch in diameter, was placed parallel to the former at 2J inches 
greater distance from the pillar. 
At the beginning of 1868 a wooden hut, three of whose sides are in great part glass, 
was erected in a retired part of the garden, and so placed that the shade of overhanging 
trees might protect it from the sun in summer, whilst it remained fully exposed to the 
sun’s rays when at low altitudes. No iron was in the vicinity at the time, but a 2-inch 
iron pipe was laid down in October 1869 at a distance of 51 feet 9 inches from the stone 
pier on which the observations are taken. 
A series of observations of the Dip, Declination, and Horizontal Force have been made 
on the same days at the old and new stations for the purpose of determining the correc- 
tions to be applied for change of locality. The corrections thus found for the first 
station are — 0*0051 in British units for the Horizontal Force, and +3' 34" for the Dip. 
It is now impossible to determine absolutely what part of these corrections is due to the 
several masses of iron near the old pillar ; but since it seems probable that the two iron 
pipes would have produced at most but a very slight disturbance, the whole correction 
has been applied from the time of the placing of the iron railings, i. e. from July 1864 
to March 1868, both inclusively. 
The following Tables contain the corrected values. 
