SURVEY OF THE BRITISH ISLES FOR THE EPOCH JANUARY 1, 1891. 
7 
Ej^och. 
The epoch of the survey is January 1, 1891, to which date all the observations 
made have been reduced. 
Magnetic Units. 
In our previous survey we employed metric units in which the value of H in the 
south of England is about 1‘8, the decimal place in the expression for a magnetic 
force being one place further to the right than in the C.G.S. system, in which 
H = OH8 (about). At that date the metric system was in use both at Greenwich 
and Kew. There is no doubt that in future C.G.S. units Avill be almost exclusively 
employed, but it would obviously be inconvenient to use two systems in two papers 
so closely related as this and our previous memoir. 
We therefore adhere to the metric system, but wherever doubt is likely to arise, 
we either draw attention to the fact, or give the value of the quantity under dis¬ 
cussion in both systems. 
Instruments. 
The instruments used were four Kew-pattern Magnetometers, by Elliott Bros., 
viz.. Nos. GO, 61, 70. 73. Of these, 60 and 61 had been used in our earlier survey 
by Professors Pucker and Thorpe respectively, and as their constants were well 
known they were assigned to the assistants. Nos. 61 and 70 belong to the Depart¬ 
ment of Science and Art. No. 73 was purchased out of the Poyal Society grant. 
No. 60 was used throughout by Mr. Gray, No. 61 in 1890 by Mr. Briscoe, and 
thereafter by Dr. Thorpe. 
No. 70 was used by Professors Rucker and Thorpe in 1890, and by Mr. Watson 
in 1891 and 1892. 
No. 73 was used by Professor Rucker in 1892. 
Four Dip Circles were also employed, all of which were made by Dover, of Charlton, 
in Kent. 
Nos. 74 and 83 were used in the original survey. Of these. No. 74 was employed 
by Mr. Gray in the years 1890, 91, 92. During the summer of 1892 Needle No. 2 
was injured and Needle No. 1 of Dip Circle 83 was sent to Mr, Gray. A new needle 
was then made to replace the injured one and was therefore used with Dip Circle 83, 
being called No. 3. 
Dip Circle 83 was used by Mr, Briscoe in 1890 with Needles 1 and 2, and by 
Dr. Thorpe in 1892 with Needles 2 and 3. 
Of the two new circles. No. 94 was used by Professors Rucker and Thorpe in 
1890 and by Mr. Watson in 1891-92. 
No. 99 was used by Dr. Thorpe in 1891 and by Professor Rucker in 1892. 
