494 
POPULAR SCIENCE REVIEW. 
meter also resembles Rutherford's. During the process of 
construction, a small piece of glass is passed into the tube, 
which nearly fills the bore : this piece of glass is pushed 
down nearly to the bulb, and then the tube is bent at that 
part. When the temperature rises, the mercury is forced past 
the obstruction ; but when the temperature falls, it cannot 
return, the contraction in the mercury occurring below the 
bend.* Thus the further end of the detached column shows 
the limit ’of maximum rise. When a reading has been taken, 
the instrument is reset by inclining it, bulb downwards, and 
giving it a gentle shake. 
A good mercurial minimum self-registering thermometer was 
long a meteorological desideratum ; for it is obvious that com- 
parisons of temperatures determined with two such opposite 
substances as mercury and alcohol could not possibly be en- 
tirely satisfactory. 
In various experiments conducted by Mr. L. M. Casella for 
this purpose, it occurred to him that the adhesiveness of 
mercury for glass in vacuo— together with the fact that 
where two tubes are united to one bulb, this fluid will rise by 
expansion in the larger and recede by contraction in the 
smaller tube — might enable him to obtain the object in view. 
The general form and arrange- 
ment is shown in the annexed figure. 
b is a tube with a large bore, at the 
end of which a flat glass diaphragm 
is formed by the abrupt junction of 
the small chamber, a b, the inlet to 
which at b is larger than the bore of the main tube. The 
result of this is, that on setting the instrument, the contract- 
ing force of the mercury in cooling withdraws the fluid from the 
main tube only, whilst on its expanding by heat, the long 
column does not move, the increased bulk of mercury finding 
an easier passage ; namely, into the small pear-shaped chamber 
attached. 
e 
f 
To set the instrument, place it in a horizontal position, 
with the plate e suspended on a nail, and the lower part 
* The contraction in the portion cut off is too insignificant to require to be 
taken into consideration. 
