508 DR. LAMONT ON THE MAGNETIC OBSERVATORY 
c. The measurement of absolute intensity requires much la- 
bour and great precision, and the results are at last uncertain to — 
nearly the one-hundredth part of the amount. 
I pass by several arrangements by which I tried to remedy 
these defects, as in the course of a different inquiry I soon 
arrived at results which made it unnecessary to proceed with 
them. Being aware that different observatories used bars of 
very different magnitudes, and that whilst very different opinions 
prevailed as to the necessity of using large bars, no experiments 
had been made to determine whether any and what differences 
were occasioned by difference of size, this point appeared to 
me to require further elucidation. I therefore procured from 
Gottingen four bars of 25 Ibs., two of 10 lbs., and two of 4 lbs., 
being otherwise provided with smaller bars. I first suspended a 
25 lb. bar in the magnetic observatory, and a 4 1b. one in the 
astronomical observatory, and had them observed simultaneously. 
The result was that the two instruments showed a march nearly 
parallel for a short time; but on continuing the observations 
considerable differences always presented themselves. ‘This re- 
markable result appeared to show still more strongly the neces- 
sity of continuing the investigation. I next constructed a small 
declinatorium (figured below), with a magnet weighing only 
1 gramme, intending to compare its results with those of the 
25 lb. bar, in the expectation that the great difference of size 
between the two instruments would render the difference of their 
march much more striking. 
mm is a small magnet bearing in the 
middle the mirror s, and itself suspended 
by a silk fibre ¢ c, placed under a bell- 
glass perforated at the top and suspended 
by a brass wire a. A piece of the bell- 
glass was cut out at p, and a plane glass, 
through which the mirror could be 
viewed, was inserted instead. The bell- 
glass was closed below by a round plate 
666, and made air-tight. The bottom 
plate was first made of copper, on ac- 
count of its quieting effect on the magnet, 
a purpose which appeared fully answered, 
as the instrument was quite free from 
oscillation. Subsequent experiments induced me to take away 
