84 REPORT—1885. 
Lt; ~— Ia 
from the proposed international discussion, and left entirely to specialists 
for snbsequent treatment. , 
With regard to the discussion of disturbances, we would suggest that 
each expedition should draw up a list of the days, selected according to : 
Gottingen time, considered by them a disturbed day, and then from a 
comparison of such lists the Conference should decide on what days 
should be selected for particular discussion in addition to the term days. 
Question 3.—Dr. Wild’s suggestion as to plotting the curves is so 
very convenient that we have already adopted it in making preliminary 
curves of the Fort Rae observations. It will be necessary in addition, 
however, to decide upon the scale of abscissee to be used for the 20- 
second interval observations on term hours. We suggest the employment 
of a scale giving six minutes of abscissee to each minute of time. 
Questions 4 and 5.—The conversion of Gaussian units into those of 
the C.G.S. system is so simple that it is unnecessary for the Conference — 
to disturb the existing historic system. The Kew Observatory has already 
for years published their results in both systems. The foot-grain system ~ 
is rapidly becoming obsolete, most magnetometers now constructed having 
métre instead of foot scales. 
XI. Letter from General Lefroy to Professor Stewart. 
82 Queen’s Gate, S.W.: 
July 15, 1885. 
My dear Professor,—I have carefully read, and return herewith, the 
papers of Senhor Capello and Dr. Wild. I have difficulty in attaching — 
a physical idea to the ingenious method of projection proposed by Senhor — 
Capello. He gives the movement, projected on a plane perpendicular to 
the dip of the axis or intersection of the plane of dip and the plane of 
declination; but I do not see how the variations of total force are to be — 
shown in conjunction with this, or with what physical notions to connect — 
the resulting curves. The actual realisation of the suspension of a 
needle by its centre of gravity without friction in any direction, especi- 
ally if counterpoised to carry a mirror, would be a great achievement, — 
but, with great respect, I doubt its being possible. Still his comparison — 
of Lisbon and Pawlowsk is very curious, and strongly confirms my belief 
that, be our stations few or many, the results at all of them must be 
brought into one view, by identity of treatment and prompt circulation, 
to obtain a clue, and to effect this we want a Deus ex machina. i 
My file of bulletins of the International Polar Commission does not go_ 
beyond Part 5. I have not seen Herr van der Stok’s communication, which — 
Dr. Wild refers to. It has occurred to me, following a hint of Lloyd’s,! 
that the area of movements would be a good measure of the forces pro- 
ducing them, and that it might be possible by an instrument on the prin- 
ciple of Amsler’s planimeter to integrate these areas for the whole 
twenty-four hours, or any not very small portions of it, in moderate dis- 
turbances. The extremely active ones would not be easily measurable. 
To take cognisance, as has sometimes been done, of those movements 
only which coincide with hours of mean time or Gottingen time, appears 
to me to forego the special advantages of continuous record. I agree 
with Dr. Wild that there is no difficulty in selecting the normal days at 
1 Trans. R.I.A., vol. xxii. 
