inches in length, and the diameter of their bore 
should be at least 2 or 24 lines, otherwise the 
friction, and capillary action, will be apt to affect 
the free motion of the mercury. The glass should 
not be very thick, as it is apt, in that case, to 
break, when the mercury is boiled in the tube: 
half a line is sufficient. 
The mercury ought to be perfectly pure, and 
free from all foreign metals. The best is what 
has been recently revived from cinnabar; the 
common mercury of the shops being often adul- 
terated intentionally with tin, lead, and bismuth, 
stands at various heights in the tube, according 
to the nature and quantity of the foreign sub- 
stances with which it is amalgamated. The 
different mechanical methods which have been 
proposed for purifying mercury, are, for the most 
part, ineffectual; we would, therefore, recommend 
the revivification of the metal from cinnabar, for 
nice barometers, as being deast liable to uncer- 
tainty. For this purpose, take a pound of cin- 
| nabar, and reduce it to powder; mix it well with 
| five or six ounces of iron, or steel filings; and 
| having put the mixture into an iron retort, 
expose the whole to the heat of a reverberatory 
furnace; the mercury will soon pass over in a 
state of great purity, and may be obtained by 
adapting to the retort an earthen receiver which 
has been previously half filled with water. Be- 
fore being introduced into the tube, the mercury 
ought to be well heated, or even boiled in a 
glazed earthen pipkin, in order to drive off any 
moisture which may adhere to it; but this will 
be unnecessary, if the mercury has been recently 
revived. 
The mercury ought likewise to be boiled in the 
tube, to expel any air or moisture which may 
still remain attached to it, or to the inside of the 
tube. This is done in the following manner : 
Pour as much mercury into the tube as will make 
it stand to the height of three or four inches; 
and introduce along wire of iron to stir it during 
the act of boiling.. Expose the mercury in the 
tube gradually to the heat of a chafing dish of 
burning charcoal; and when it begins to boil, 
stir it gently with the iron wire to facilitate the 
disengagement of the bubbles of the air. When 
the first portion of the mercury has been suffi- 
ciently boiled, and all the air extricated, remove 
the tube from the chafing dish, and allow the 
whole to cool, taking care not to bring it into 
contact with any cold substance. Introduce an 
equal quantity of mercury, and treat it in the 
same manner, withdrawing the wire a little, so 
that it may not reach below the upper part of 
the mercury already freed from air. The chafing 
dish must also be placed immediately under the 
mercury which has been last poured in. Repeat 
the same process with each successive portion of 
mercury till the tube is filled, always applying 
the heat very cautiously; and be equally careful 
in allowing it to cool, before a fresh portion of 
mercury is poured in. 
BAROMETER. 
309 
It sometimes happens, when the tube is care- 
fully inverted, as in the Torricellian experiment, 
that the mercury, after being completely freed 
from air, in the way we have described, remains 
suspended in the upper part of the tube, and does 
not assume its proper level, with respect to the 
pressure of the atmosphere, till the tube has been 
gently shaken. This fact—which seems to have 
given great difficulty to Huygens—is certainly 
owing to the capillary attraction of the tube, 
and the mutual attraction of the particles of 
mercury, as it takes place only in tubes of a small 
bore. To say that it is owing to the influence 
of an invisible ethereal fluid more subtile than 
air, is no less unphilosophical than the semicir- 
cular hypothesis of Linus, or the principle of the 
horror of a vacuum, particularly when we can 
assign a cause for it, of whose operation we have 
many simple and obvious proofs. 
The common barometer, represented in J/g. 2, 
Plate VII., differs but little from the Torricellian 
tube. Instead of a bason, a small reservoir is 
usually attached to the lower extremity of the 
tube, or rather the tube itself is swelled out into 
a bulby form, as represented at Hg. 3. In this 
form, however, the instrument is by no means 
accurate, particularly when the diameter of the 
bulb, as is usually the case, differs but little from |, 
the diameter of the bore of the tube. In order 
to keep the surface of the mercury in the bason 
always at the same level, the father of the late 
Mr. George Adams constructed the bason wholly, 
or in part, of leather, and by means of a screw at 
the bottom, adjusted the surface of the mercury 
in it, so as to have it always at the place from 
which the divisions on the scale commence. In 
this country, the lowest station of the mercury 
is observed to be about 28 inches, and its highest 
31 inches above the level of the mercury in the 
bason: and when the instrument is to remain in 
a fixed position near the surface of the earth, we | 
may consider the interval between these two 
points as the range of the barometrical scale. 
The scale which consequently will embrace three 
inches, may be subdivided into smaller divisions, 
according to the degree of nicety required. Each 
inch is commonly divided into ten equal parts ; 
and these are subdivided into hundredths of an 
inch, by a contrivance called a vernier scale. 
By this means the height of the mercury is as- | 
certained, by inspection, to the 1gcth ofan inch..- 
For nice purposes, the vernier may be made to 
indicate the roooth of an inch. 
Besides these lineal divisions, the scale is 
divided into other points, having a reference to 
the instrument in its capacity of indicating the 
probable state of the weather. At 31 inches, the 
highest point of the scale of variation, set fair is 
marked on the one side, and set frost on the other. 
At 30 inches, fatr is written, in like manner, on 
the one side, and frost on the other; and at half 
an inch below is written the word changeable, 
which answers both for summer and winter. 
