76 REPORT—1885. 
stituents. My meaning is that we ought to express the result in sets of 
terms of this form. 
Ay+A,cos6+A,cos29+ ... re: 
+a,sinO6+a,sin26+.... $ 
By +B, cos 64+ B, cos 20+... ee 
+b, siné6é+b,sin20+.... p 
I had some time back a letter from Chambers at Bombay in which he 
says that he considers he has detected a lunar inequality. Now, unless 
this is certainly incorrect, is it not desirable to submit the quantities to 
analysis according to lunar time? I take it that your proposal as to 
spherical harmonic representation is to put the Ag, Aj, As, a, de, &e., 
as constants multiplied by spherical harmonic functions of the latitude 
and longitude of the place of observation, Gauss had, as I fancy, only 
considered the mean values in this way, and you are proposing to treat 
the diurnal inequality in a similar manner. 
If much harmonic analysis is to be done, some form nearly like that 
used for tidal reductions would seem to be useful. 
The chief complication of those forms consists in the fact that the 
tide-heights are taken at exact solar hours, whereas we want measure- 
ments taken also at mean lunar and a number of other kind of hours. 
All this is avoided in your case, unless indeed you carry out an analysis 
for the alleged lunar influence. 
Yours sincerely, 
G. H. Darwin. 
V. Notes on the above Suggestions. By Professor Batrour Stewart. 
The suggestions of the Committee are invited upon the following 
points : 
1. Do they agree with the suggestion of Dr. Schuster, that it is of 
importance to ascertain the solar-diurnal variation of the three magnetic 
elements at various stations of the earth’s surface, with the view of treat- 
ing these after the method of Gauss ? 
2. Assuming that observations made at stations near the magnetic 
pole need special treatment, do the Committee think with Sir Henry 
Lefroy that even in ordinary localities the method of Sabine is objection- 
able for obtaining a correct value of the solar-diurnal variation? Asa 
good many declination observations have been treated by this method it 
is of importance to set the question at rest, and the suggestions of the 
Committee are invited as to the best means of doing this. 
3. What do the Committee think of the herein-recorded method of 
obtaining the solar-diurnal variations in the case of the horizontal and 
vertical force instruments? I may state that a point of immediate 
scientific importance arises regarding the V. F. solar-diurnal variation, 
inasmuch as the observers at Lisbon and Bombay suspect that this, unlike 
the diurnal variations of the other two elements, does not vary with the 
state of the sun’s surface. It would be very desirable to obtain con- 
clusive evidence of this from other stations, 
