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IV. A Contribution to Terrestrial Magnetism ; being the Record of Observations of 
the Magnetic Inclination, or Dip, made during the Voyage of H.M.S. 4 Iron Duke 5 
to China and Japan, &c., 1871-75. By Vice-Admiral Sir Charles Shad well, 
K.C.B., F.M.S. 
Received and Read June 15, 1876. 
In the following paper I propose to place on record, and submit for the information of 
the Royal Society, the observations made by me for the determination of the magnetic 
inclination, or dip of the magnetic needle, during the recent voyages of H.M.S. 4 Iron 
Duke ’ in the Eastern Seas, when visiting China, Japan, and places adjacent, between 
October 1871 and April 1875. 
The instrument employed was a six-inch dip-circle by Robinson, furnished with two 
needles, and graduated to ten minutes, which was formerly the property of the late 
Capt. Francis P. Blackwood, R.N., and which was used by me during that officer’s 
survey of the N.E. coast of Australia in H.M.S. 4 Fly,’ 1842-46. 
This instrument was subsequently lent to me -while I was in command of H.M.S. 
‘Highflyer,’ employed on the China Station, 1857-59, and has since passed into my 
possession by gift from the son of my former Captain. 
I have thus had the opportunity of repeating, with the same circle and needles, the 
observations made at various places in China and elsewhere after the lapse of some 
years, and have thus obtained data for the approximate deduction of the values of the 
secular changes in this element at some of the stations visited, which may perhaps give 
to these observations a greater value than they would otherwise possess. 
Moreover, in some instances, I have not only had the opportunity of observing the 
dip at stations along the eastern coasts of Asia, but also of obtaining its value at some 
places in the chain of islands lying to the eastward from Labuan up to Yezo, which will 
thus afford the means, by aid of a graphic projection, of determining, approximately, 
the general direction of the 44 isoclinal lines ” in those parts. 
The observations were made generally in the manner described in the 4 Admiralty 
Manual of Scientific Inquiry,’ with every possible precaution to eliminate accidental 
errors of observation ; the readings were taken both for the lower and upper ends of the 
needle, on the graduated circle, 44 face to face ” and 44 face reversed,” 44 face east ” and 
44 face west,” 44 poles direct ” and 44 poles reversed,” three readings being taken in each 
position of the instrument, so that the final result for the dip for each needle is the 
mean of forty-eight readings ; and the observations being repeated in like manner with 
the second needle, the 44 mean concluded dip ” at each station is the mean result of the 
observations by the two needles. 
mdccclxxvii. x 
