te ee ie pi a ae 
PRELIMINARY REPORT ON TERRESTRIAL MAGNETISM. 15 
which has been provided at De Bilt by the Meteorological Institute of the 
Netherlands, at present under the directorship of Professor van Ever- 
dingen. This scheme is so well known that a brief reference to it will 
suffice. All co-operating observatories send in a quarterly list in which 
each day has assigned to it the ‘ character ’ figures 0, 1, or 2, according as 
the magnetograph curves are regarded as quiet, moderately disturbed, or 
highly disturbed. Using these returns, the director at De Bilt assigns a 
mean ‘character’ figure to each day, and selects for each month the 
five days which seem the best representatives of quiet conditions. He 
also selects a variable but smaller number of highly disturbed days. It is 
hoped that all co-operating observatories will derive and publish diurnal 
inequalities for the five monthly ‘ quiet’ days, and also that they will 
publish copies of their curves for some at least of the selected disturbed days. 
A good many observatories do both these things. Some publish diurnal 
inequalities from the ‘ quiet’ days alone, but most which publish ‘ quiet’ 
day inequalities publish other inequalities as well. An international 
“quiet ’ day, it is important to notice, is a twenty-four hour period com- 
mencing at Greenwich midnight, and international ‘ quiet ’ day inequalities 
thus refer to exact hours (G.M.T.). One of the recommendations of the 
International Committee, which has not been universally acted on, has 
been that magnetic observatories should employ local mean time (L.M.T.). 
Any observatory, however, other than Greenwich, which employs L.M.T., 
or any time which differs from G.M.T. by fractions of an hour, has to take 
two different sets of measurements if it adopts the international ‘ quiet’ 
day system. The amplitude of magnetic disturbance varies immensely at 
different stations, and the attachment of ‘ character ’ figures is so arbitrary 
an operation, that the standard is apt to vary considerably, even at the 
same observatory. Thus a desire to replace the existing method of 
discriminating between days by something based on exact numerical 
_ measurement has been somewhat widely felt. A scheme based on what its 
author, the late Professor Bidlingmaier, of Munich, called ‘ magnetic 
{ activity ’ seemed to present possibilities, and the International Bureau 
appointed a sub-committee consisting of Professors van Everdingen and 
Schmidt, and the writer, to consider it. A discussion of the subject by the 
writer, based on a comparison of curves from more than twenty obser- 
vatories, appeared in the journal Terrestrial Magnetism for June 1917. 
along with a similar article by Mr. D. L. Hazard. 
Another scheme set going by the International Committee relates to the 
intercomparison of the absolute magnetic instruments of different countries. 
The scheme was that each of the principal countries should, in succession, 
undertake the duty of observing at foreign observatories, with a view to 
finding the difference between its own standard instruments and those of the 
countries visited. One or two such sets of comparisons have actually 
- been made. 
$2. The second agency mentioned above, the Department of Terrestrial 
‘Magnetism of the Carnegie Institution of Washington, is in some ways the 
antithesis of the first. It is not international, its policy is largely that 
of a single man, the Director, and it has large funds at its disposal. It 
is thus an institution which does not primarily exist for ventilating opinions, 
but for getting things done. Hitherto, it has chiefly concentrated on a 
_ single problem, the execution of a general magnetic survey of the earth, but 
there are indications of the development of other lines of activity. 
