130 
MAJOR SABINE ON TERRESTRIAL MAGNETISM. 
contributed to this map are the series of Lieut. Sulivan above referred to, and a series 
by Mr. James Dunlop of the Paramatta Observatory. The position of the lines over 
the surface of the land rests on above 140 determinations made on shore, within the 
limits of the map, between the years 1834 and 1839, of which nearly the half are now 
first published. 
The series of Messrs. Sulivan and Dunlop include observations of the intensity 
made at sea, as well as of the inclination. I have made the necessary computations 
to reduce their results to the relative scale in general use. Previously, however, to 
any remarks which the consideration of these results may suggest, it will be proper 
to give some account of the observations from which they are derived. 
Lieut. Sulivan’s series was made during a voyage from Falmouth to the Falkland 
Islands in H.M.S. Arrow in 1838; from the Falkland Islands to Rio de Janeiro in 
H.M.S. Stag in 1839 ; and from Rio back to Falmouth in the packet-ship Opossum 
in the same year. The instrument employed by him was a dip circle of four inches 
diameter, made by Mr. Thomas Jordan of Falmouth, furnished with two needles, one 
for the dip and the other for the intensity, on the plan devised by Robert Were Fox, 
Esq., and described in the third volume of the Annals of Electricity, &c. The dip 
was in all instances observed directly, without the aid of the deflecting magnets. 
The values of the intensity were obtained by means of the angles of deflection from 
the dip, produced by the employment of constant weights applied to the axle, correc- 
tions being made for differences of temperature in the manner pointed out by Mr. 
Fox. The Table, Appendix A, which has been drawn up by Lieut. Sulivan, con- 
tains the particulars of the dip observations, and of the geographical positions to 
which they belong; the weights used on each occasion for the intensity; the angles of 
deflection observed, and the same corrected for temperature; the values of the inten- 
sity resulting from the experiments with each weight, and the mean values : these 
are given relatively to unity in Mr. Fox’s garden at Falmouth, which was the base 
station of Lieut. Sulivan’s observations, and, in the final column, in terms of the 
relative scale in common use, in which the force in London = 1*372, and at Fal- 
mouth 1'374L 
The circumstance of primary consideration in a series of intensity observations of 
this nature is the degree of steadiness with which the needle may have preserved its 
magnetic condition. The following observations, made in Mr. Fox’s garden at Fal- 
mouth before and after the voyage, show that no notable alteration took place in this 
respect during its employment. 
* Magnetic Survey of the British Islands, British Association Reports, 1838, p. 192. 
