AND METEOROLOGICAL INSTRUMENTS, BY PHOTOGRAPHY. 
23 
and consequently the statical moment of the mercury displaced between the tem- 
peratures of 32° and 32°-f-c^’° will be (J{cx-\-kex^) w; let now v be the weight wfiieh, 
placed at a unit of distance from the point of support, will represent the temp(;rature 
correction for 1° above 32° Fahr. ; it will only remain to obtain a bulb of such size that 
kw—v; 
we shall then clearly have the statical moment of the mercury displaced from the 
bulb by .^’° of elevation of temperature above 32° Fahr., and transferred to the vacant 
portion of the tube represented by 
{cx-\-ex^) V, 
and consequently a correction in weight equivalent to the temperature coefficient 
will have been applied to the bar. 
As the change of density of the thread of mercury in the stem has not been taken 
into the account, it will be better to place the point of the tube which corresponds to 
a distance ^ kc below the point 32° opposite the knife-edge ; the error from this 
cause will then be quite negligible. 
The value of v is to be determined experimentally by observing the displacement 
of the register line occasioned by a small known weight placed on the bar at a known 
distance from the point of support, and comparing this with the scale coefficient ob- 
tained in the usual manner. 
Owing to the short period of time that has elapsed since the completion of the in- 
struments, and the difficulty of making accurate magnetic observations in a locality 
subject to the constant tremors and vibrations of a London thoroughfare, the 
constants have not been determined with a sufficient degree of accuracy for publica- 
tion ; when satisfactorily determined, they will be communicated to the Society in 
the form of a Supplement to this paper. 
