THE EXHIBITION 01' 1862. 
77 
necessary for an observer to be constantly in attendance, in 
order to register tlie different fluctuations of the magnets. 
Mr. Brook hit upon the happy idea of applying the photo- 
graphic process to this end, and at the last Exhibition his 
apparatus for this purpose was in full work. Similar ma- 
chinery, as arranged by the late Mr. Welsh, and now in 
use at the Kew Observatory, is here exhibited, and it will be 
seen with what simplicity the light coming from one fixed, 
and another hemispherical mirror, attached to the magnets, 
impresses two images on a waxed paper photographically pre- 
pared — one of which gives a fixed line, and the other, of course, 
moves according to the oscillations of the magnets, the dis- 
tance between the two showing the amount of the latter. This 
principle has also been applied to registering the fluctuations 
of the barometer and thermometer, and is preferable to any other, 
— in the former case, being much more safe and simple than 
the numerous self-registering mechanical apparatus here exhi- 
bited, for showing the height of the mercury during the twenty- 
four hours, in all of which friction takes place more or less. 
The ozonometer of Dr. Lankester is an ingenious adap- 
tation of the same self-recording principle, only that, in this 
case, it is the life-giving properties of the atmosphere (and 
not solar or artificial light) which leave traces on the prepared 
paper. The effect of this element is shown in a large diagram, 
drawn up by the same authority, representing the various tints 
of slips of paper exposed during different states of the 
weather; and to the meteorologist this is as useful, and 
should be as strictly attended to, as the weight, tempera- 
ture, or humidity of the atmosphere. To those who have 
used the ordinary maximum thermometer, with an index 
floating on the mercury, which is only too liable to get out of 
order, the self-registering maximum thermometer of Professor 
Phillips, consisting of a broken column of mercury, will be a 
great boon. The mercurial minimum thermometer, of Casella, 
is another great improvement ; 
in this case advantage is taken 
of the adhesive property of mer- 
cury for glass in vacuo, and also 
of the fact, that when two tubes 
are joined to one bulb, the mer- 
cury will rise in the larger by expansion, and fall in the 
smaller by contraction. He therefore makes a branch, d, to the 
ordinary thermometer, having a flat diaphragm at b, the inlet 
to which is larger than the bore of the long tube, c. The mer- 
cury in cooling withdraws the fluid in the indicating stem only, 
whilst on expanding, itflows into the passage where it finds the least 
resistance ; in other words, the mercury in the long tube cannot 
