426 FIRST REPORT UPON MAGNETIC WORK

1857-62. In the Philosophical Transactions of the Royal Society for
1870 will be found a full account of this survey and likewise of the
earlier one. In this paper Sir Edward Sabine combined the observa-
tions of the two surveys and drew the isomagnetic lines for the mean
epoch of 1842-5. And now again Great Britain has just completed a
third magnetic survey, far more elaborate than any of the preceding
ones. This survey, one of the most carefully executed up to date,
was conducted by two most eminent physicists, Professors Rucker and
Thorpe. ¹ It was a most fruitful piece of work. Observations of the
three elements were made at first by the two distinguished professors
themselves between the years 1884—88 at 205 places. ² The Govern-
ment Grant Committee of the Royal Society then made a liberal
grant so that the survey might be carried out on a larger scale than
hitherto attempted. Two assistant observers were then employed,
and with their aid, in the four years 1889-92, the grand total of the
number of stations was brought up to 882, making, on the average, one
station to every 139 square miles of land area. ³ The isomagnetic lines
corresponding to the epoch 1886-90, and based on the 205 observa-
tions made between the years 1884-88, were drawn, and likewise those
as based on the 677 stations observed in 1889-92, were constructed for
the epoch 1891, and finally the lines as resulting from all the stations
were obtained. A splendid opportunity was thus afforded for testing
the accuracy with which the positions of the isomagnetic lines, e. g.,
the lines of equal magnetic declination or variation, can be inferred
from observations in greater or less number. We shall have occasion
to revert to this particular matter in a future report. In the mean-
while we refer the reader to Professor Rucker's interesting account⁴
published in Terrestrial Magnetism for July, 1896.

¹ To Dr. Thorpe we owe a number of determinations of the magnetic
elements in our own country.

² The results were published in the Phil. Trans. of the R. S, 1890, A, p. 53,
the memoir constituting the Bakerian Lecture of that year.

³ The results of this last work have just been published, Phil. Trans.
R. S., vol. 188, A, 1896.

⁴ A. W. RUCKER: A Summary of the Results of the Recent Magnetic
Survey of Great Britain and Ireland conducted by Professors Rucker and
Thorpe: —I. On the Accuracy of the Delineation of the Terrestrial Isomag-