478 
NATURE 
[ Sepz. 16, 1886 
Considering the Best Means of Comparing and Reducing Mag- 
netic Observations. Drawn up by Prof. Balfour Stewart.—lt 
is with deep regret that the Com nittee record the death of one 
of their number—Capt Sir Frederick Evans, so well known for 
the valuable contributions which he had made to terrestrial 
magnetism. His eminent scientific qualities combined to make 
him a greatly esteemed member of this Committee, who now 
deplore his loss. 
The Committee have added to their number the following 
gentlemen :—The Astronomer-Royal, Mr. William Ellis, Prof. 
W. G. Adams, and Mr. W. Lant Carpenter. They could hardly 
consider their list complete without the addition of the first two 
names, and they are glad that, although not members of the 
British Association, these gentlemen were not unwilling to serve 
on one of its Committees. 
Since the last meeting of the Association Mr. G. M. Whipple 
has made a comparison between the method «f obtaining the 
solar-diurnal variation of declination adopted by Sir E, Sabine 
and that of Mr. Wild. These methods were applied to three 
years’ observations at the Kew Observatory, and the results were 
compared with those deduced by the Astronomer-Royal from 
the same three years at Greenwich. The comparison will be 
found in Appendix IV. to this report. 
The Committee think that this comparison deserves careful 
study, but they do not feel themselves able to pronounce as yet 
upon the comparative merits of these various methods. Never- 
theless, they are of opinion that it is hizhly desirable to record 
the daily mean values (undisturbed) of the three magnetic ele- 
ments side by side with their solar-diurnal variations. 
It will be seeen by Appendix III. that Sir J. Henry Lefroy 
has continued his comparison of the Toronto and Greenwich 
observations. He has obtained from the smooth curves—that is 
to say, taking Mr. Wild’s method—results which appear to 
show thar the turning-point of the declination is decidedly later 
in local time at Toronto than at Greenwich. Sir J. H. Lefroy 
attributes this to the fact that these two stations are on different 
sides of the Atlantic.1 
Appendix II. exhibits, by aid of a diagram, an interesting 
comparison of Senhor Capello between the diurnal variation of 
the inclination and that of the tension of aqueous vapour. It 
is remarkable to notice the great similarity between these varia- 
tions ; a similarity which holds separately for each month of the 
year. Senhor Capello hopes that these results may be confirmed 
by a more extended series of observations. 
The researches to which allusion has now been made refer to 
the solar-diurnal variation, excluding disturbed observations. 
With respect of disturbances, Sir J. Henry Lefroy has con- 
tinued his comparison of Toronto and Greenwich, and his results 
are indicated in Appendix ITI. 
Prof. W. G. Adams has, it is well known, made, in connec- 
tion with another Committee, extensive comparisons between 
the simultaneous traces of magnetographs in various places. 
He is at present engaged on such an undertaking, and the Com- 
mittee are in hopes that when this is completed he will give 
them the benefit of his experience. The subject is an extremely 
interesting one, and it seems not impossible, judging from the 
Greenwich results as obtained by Sir G. B. Airy, that magnetic 
disturbances may be in a great measure due to earth-currents, 
so that an easy approximate method of recording the latter may 
be obtained from magnetograph indications. 
The Rev. S. J. Perry and Prof. Stewart (Appendix V.) have 
completed their preliminary comparison of certain simultaneous 
fluctuations of the declination at Kew and at Stonyhurst in a 
paper which has been published in the Proceedings of the Royal 
Society, No. 241, 1885. The results are virtually those which 
were stated in the last report of the Committee. The comparison 
is being continued and extended. 
Prof. Stewart and Mr. W. Lant Carpenter (Appendix VI.) 
have given the results of other four years’ reduction of Kew 
declination disturbances classified according to the age of the 
moon. ‘These are very similar to the results of the first four 
years given in our last report. The same observers give a com- 
parison, extending over four years, between declination disturb- 
ances and wind values, which appears to them to show that 
there is some relation between these tvo phenomena. They are 
anxious to continue and extend both these inquizies. 
Prof. Stewart has pointed out certain general considerations 
which appear to indicate that the solar-diurnal variation may 
perhaps be caused by electric currents in the upper atmospheric 
* See Appendix by Sir GB. Airy to the Greenwich Observations, 1884. 
regions. Dr. Schuster has likewise made a preliminary applica- : 
tion of the Gaussian analysis, tending to confirm the hypothesis 
that currents in the upper regions are the cause of these 
variations. ! 
By this analysis Dr. Schuster obtains certain relations between 
the solar-diurnal variations of the three magnetic elements which — 
ought to hold on the hypothesis that these variations are caused 
by currents in the upper atmospheric regions. One of these is 
that the horizontal force component of the daily variation ought 
to have a maximum or minimum at the time when the declination 
component vani-hes—that is to say, atlains its mean position, 
Another is that the horizontal force ought to be a maximum 
in the morning and a minimum in the afternoon in the equatorial 
regions, while in latitudes above 45° the minimum ought to take 
place in the morning. A third is that in the equatorial regions 
the maximum of horizontal force ought to be coincident with 
the minimum of vertical force, and vice versa. 
These conclusions are sufficiently well confirmed by observa- 
tions, and thus render hopeful the first attempt to apply the 
Gaussian analysis to the solar-diurnal variation. 
The appendices of Capt. Creak (I.) and of Dr. Schuster 
(VII.) have reference to this subject, and indicate the import- 
ance of some action being taken by the Committee to prepare 
for a thorough application of the Gaussian analysis to the mag- 
netic variations. It will be seen from the remarks of Dr, 
Schuster that some time must elapse before observations are 
obtained sufficiently good and complete to justify a systematic 
application to them of mathematical analysis. This circum- 
stance has induced the Secretary to lay before this Committee 
in Appendix VIII. a provisional working hypothesis regarding 
the cause of the periodic variations of terrestrial magnetism 
which has gradually grown up by contributions from various 
quarters. 
While this Committee do not hold themselves - responsible for 
the various statements contained in this hypothesis, they would 
point out the desirability of ascertaining to what extent well- 
known magneto-electric laws may succeed in accounting for the 
phenomena of terrestrial magnetism, and likewise the desirability 
of ascertaining to what extent the magnetic earth appears to be 
subject to the laws of ordinary magnets. 
A preliminary working hypothesis of this nature might serve 
to elicit facts while the material for the Gaussian analysis is being 
completed, and it would add to the interest of the final result if 
we should obtain reason to think that electric currents in the 
upper atmospheric regions are at once the zmmediate causes of 
magnetic variations and the effects of atmospheric motions in 
these regions, so that a knowledge of the one set of currents 
might possibly enable us to determine the other. 
Finally, in Appendix IX. we have a list drawn up by Sir J. 
Henry Lefroy of the various stations where magnetic observa- 
tions of any importance have been made. 
The Committee have drawn 10/. Ios., and returned to the 
Association a balance of 297. 10s. They would desire their 
re-appointment, and would request that the sum of 50/. should 
be placed at their disposal, to be spent as they may think best 
on the researches mentioned in this report. 
Third Report of the Committee, consisting of Prof. Balfour 
Stewart (Secretary), Prof. Stokes, Prof. Schuster, Mr. G. Fohn- 
stone Stoney, Prof. Sir H. E. Roscoe, Capt. Abney, and Mr. 
G. F. Symons, appointed for the Purpose of Considering the Best 
Methods of Recording the Direct Intensity of Solar Kadiation.— 
The Committee, in conformity with their last report, have had 
constructed by Mr. Casella an instrument of the following 
description :—It consists of a thick-sided copper cube, one side 
of which is to be exposed to the sun. In the thickness of this | 
side are inserted two thermometers ; a third is put in the side 
opposite ; while the bulb of a fourth occupies the hollow centre. 
If the readings of these instruments are found to have any 
constant relation, the fourth instrument will be replaced by a 
flat bulb thermometer exposed to the sun’s rays through a hole 
in the sun-ward side of the cube. The Committee suggest that 
they be re-appointed, and that the sum of 20/. be again placed 
at their disposal. | 
Seetends aw Vs toly ay Seegr 
‘ 
~ me ee ee es Ser 
From the Report of a Committee, consisting of Profs. G. H. 
Darwin and $. C. Adams, for the Harmonic Analysts of Tidal 
Observations. Drawn up by G. H. Darwin.—Major Baird’s 
**Manual of Tidal Observations” is now printed, and will be 
* An account of these researches will be found in the PA/?. Jag., April 
and May 1886. 
