510 DR. LAMONT ON THE MAGNETIC OBSERVATORY 
This proof, the demonstratio ad oculum, cannot be replaced by 
any theoretic considerations, and when once obtained, it renders 
all further investigations on the subject superfluous. 
After several preliminary trials, on the 13th of May corre- 
sponding observations were begun with two similar instruments, 
one of which was placed in the magnetic observatory and the 
other in my dwelling-room. In comparisons lasting only a few 
minutes the result was perfectly satisfactory ; but when the ob- 
servations were continued, instead of the expected agreement 
notable discordances were found. 
[Observations are here detailed in illustration of these dis- 
cordances. | 
In order to investigate further the course of these remarkable 
differences, I placed in the magnetic observatory first two and 
then three instruments, which were observed simultaneously 
and gave the following series. 
[The table of observations is omitted. Dr. Lamont proceeds 
to make the following comments on them | :— 
My attention was early drawn to the circumstance that the 
differences presented a diurnal period, evidently connected with 
temperature. After different experiments on the possible effects 
of temperature, I came to facts which established the following 
principle :—Every alteration of temperature impresses on an in- 
closed mass of air a circulating movement, which once produced, 
continues for some time, though always diminishing, and keeps 
a freely-suspended magnet constantly deflected to one side of 
its true direction. ; 
This effect may be most easily shown by sprinkling a few 
drops of spirits of wine on the bell-glass of such an instrument 
as I have described. At the part where the spirits of wine have 
fallen and are evaporated, cold is produced inside the glass, the 
particles of air in contact fall down, those above follow them, and 
in the course of a few minutes a current of air is established, | 
capable of deflecting the needle at first as much as 5/, then | 
becoming gradually more feeble, but having still a perceptible 
influence at the end of three-quarters of an hour. 
After I had satisfied myself by repeated and varied experi- 
ments that the effect was produced by the current of air and by 
it alone, the application to the observed discordances in the 
different instruments was easy. By the daily rise and fall of — 
