THE MAKERSTOUN OBSERVATIONS. 
13 
expansion of the brass rod towards the north, if properly regulated, may be made 
to depress the north end of the needle as much as the diminution of the mag- 
netic moment would tend to elevate it. The expression for the proper weight and 
length of such a brass rod is more complicated than in the case of the bifilar, 
depending on the weight of the needle, and the distance of its centre of gravity 
from the centre of motion. In the case of the Makerstoun balance, for which 
^=0-00008, I have computed that a brass rod 10 inches long, aV the weight of 
the needle, fixed so that its centre of gravity should be near the centre of 
motion of the needle, would by its expansion compensate nearly for the diminu- 
tion of the magnetic moment. 
In both cases such computations could only be considered as rough guides 
to the instrument-maker who, by a few experiments at different temperatures, 
might be able to attain a more accurate compensation. 
Magnets. — My experience of small magnets is not very great, but I feel in- 
clined to prefer thin parallelopipedal magnets of a length of about six inches for 
variation instruments in a fixed observatory, to either the large or smaller 
sizes. If building them of thin bars would diminish the temperature coefficient, 
would the compound magnet be equally permanent ? For variation instru- 
ments, mirrors and scales would be preferable to collimators for such small 
magnets. 
The whole subject of the best forms and kinds of magnets, the best for per- 
manency, intensity, and smallness of temperature co-ef&cient, requires a careful 
investigation I am not aware whether any considerable use has been made of 
the labours of the Rev. Dr Scoresby, Mr Petrie, and others, for the purposes of 
an observatory. 
Instruments for Absolute Determinations. — These instruments require to be 
placed at a distance from the others, and in a place wholly free from iron. 
The Inclinometer. — This should have the needle placed apart from the reading 
circle, and as little metal should be employed as possible for the base : if the 
makers would take greater pains in obtaining metal without iron, such precautions 
would be less necessary. Such instruments as those made by M. Repsold of 
Hamburgh, with reading microscopes and short needles, seem best fitted for good 
observations. 
In order to determine the error of old instruments due to the iron in the 
circle or general structure, the following process might be employed with advan- 
tage : Connect the needle with a beam canying a lens and scale, or a mirror, 
with the scale at a distance, — suspend this by a silk thread so that the axle of the 
needle may occupy its usual position in the circle, the latter with the whole 
