ON MAGNETICAL AND METEOROLOGICAL OBSERVATIONS. 35 
would you be disposed to recommend any, and what modifications, extensions 
_ or alterations in the system of observing, or in the apparatus employed ?” 
Under Query 1, I have pointed out the preference which I would give to 
extra observations of disturbances, and have recommended the discontinuance 
at present of term observations. 
I am inclined to prefer hourly observations to two-hourly, as the first com- 
pared with the second does not add so greatly to the labour of the observer 
or the computer as might at first appear, at least when the whole work of the 
observatory is taken into account. 
IX. Dr. Lloyd to Sir John Herschel. 
Trinity College, Dublin, March 8, 1845. 
Dear Sir,—I beg to acknowledge the receipt of your letter, on the part of | 
the Magnetic Committee of the British Association, and to reply to its several 
queries as far as I am able. 
The first question, viz. “ Whether there are any, and if so, what important 
objects to be accomplished by a continuance of the existing establishments 
for a longer period,” necessarily suggests the inquiry,—how far the objects 
originally proposed have been attained, or are likely to be attained, by the 
course of observation now in progress, and terminating with the present year ? 
We may afterwards inquire, whether there are any new objects suggested by 
the results themselves, or otherwise. : 
In reference to the first inquiry, it will be convenient to keep in mind the 
distinctions of magnetic determinations into those of the absolute values of the 
direction and intensity of the magnetic force at particular epochs, and those 
of the changes which they are continually undergoing ; and again, the sub- 
division of the latter into periodic variations, secular changes, and disturbances. 
Of these, the diurnal variations are those whose determination demands the 
greatest amount of labour, and they are fortunately also those which seem 
now to be best determined. I am of opinion that five years of uninterrupted 
hourly or two-hourly observation is fully adequate to the establishment of 
their empirical laws, with all the requisite precision ; and this opinion is con- 
firmed by the examination of the results of two years at the Toronto obser- 
vatory, recently published. 
The knowledge of the annual variations of the magnetic elements, and that 
of their secular changes, can be obtained with precision only by means of 
absolute determinations, often repeated at particular epochs, and reduced to 
their mean values by the help of the differential instruments. The instru- 
mental means which we at present possess are probably sufficient to furnish 
the data required in both parts of this delicate deduction with all the neces- 
Sary exactness; but the difficulty seems to lie in the irregular fluctuations of 
the elements themselves, or in other words,‘in the want of regularity in the 
annual period, and in the progression of the secular change. ‘To eliminate 
completely the effects of these irregularities, a longer course of observation 
is probably necessary; but in carrying it on it is by no means requisite that 
the daily observations should be as numerous as heretofore. 
_ Enough has probably been done to ascertain the more obvious phenomena 
of disturbances, and perhaps also to furnish the principal data for theory. 
_ Bat it is not improbable that the application of theory may suggest or demand 
additional data respecting them, and other methods of combined observation. 
D2 
