527 
Dr. Lloyd observed that there could be no doubt of the excellence of 
the observations recorded in the foregoing communication : they afforded 
abundant proof, both of the perfection of the instruments employed by 
Professor Hansteen, and of his skill in using them. But the fact that the 
diurnal variation of the inclination has not been observed with the Eng- 
lish dip cireles could not, he thought, be fairly adduced in proof of 
their inferiority to the French instruments; inasmuch as this ele- 
ment is obtained by us more completely by other instrumental means. 
- By means of the balance magnetometer (combined with the bifilar mag- 
netometer) not only the range, but the whole course of the diurnal va- 
riation of the inclination, has been completely determined at Dublin ; 
and the same thing is true of other places at which both these instru- 
ments have been employed. In fact, it is in this circumstance that the 
advantage of the British observatories over those of the Continent mainly 
consists, the daily observations of the latter being limited to two of the 
three magnetic elements. 
Dr. Lloyd believed that it was unnecessary for him to enter further 
into the question of the accuracy attainable by the dip circles of the form 
now used by English observers, as the subject would be fully discussed 
by Mr. Stoney in a paper to be read by him to the Academy that even- 
ing. He would merely observe that Professor Hansteen seemed to be 
under a misconception in supposing that it was necessary, with these 
instruments, to observe the needle at rest. By the help of a small di- 
vided scale in the focus of each of the microscopes, the wires may be 
either placed in the position of equilibrium, while the needle itself is in 
motion, or (the microscopes being fixed) the deviation of the wires from 
that position may be observed, and added, with its proper sign, to the 
readings of the verniers. This simple addition had, in fact been »made 
in the original instrument of this construction, which was made by Mr. 
Barrow for Dr. Lloyd, and according to his specifications. 
Mr. G. J. Sronny read a paper— 
ON THE DEGREE OF ACCORDANCE WHICH MAY BE ATTAINED IN OBSERVATIONS 
MADE WITH DR. LLOYD'S DIP CIRCLES. 
In fixed magnetic observatories the instruments may be mounted with 
every appliance, however cumbersome, which contributes to accuracy ; 
but when we mean to carry an instrument from station to station, mak- 
ing observations with it at each, the necessity that it be of moderate 
size, and not easily injured or thrown out of adjustment, debars us from 
many arrangements which we might otherwise make. Yet with such 
travelling instruments very considerable degrees of accordance have been 
attained, and, as I think, much more accuracy is within our reach, by 
attention to a small mechanical detail. 
In the most sensitive of the travelling dip circles for use on shore, 
the needle rests by a thin and most carefully constructed transverse 
axis on two smooth horizontal pieces of agate, upon which the axis rolls 
without rubbing as the needle oscillates before coming to rest. A won- 
derful degree of delicate mechanical accuracy has been attained in the 
construction of this minute cylindrical axes, an accuracy which would 
