232 
A SELF-REGISTERING BAROMETER. 
heights of the columns 29‘3 and 30*5 are as- 
sumed, as limiting the space in which the 
greatest number of variations occur in the 
barometer throughout the year. 
To regulate the countervailing force, and 
calculate the power of the engine ; first, the 
countervailing force is to be adjusted to 
balance the tube in the place represented in 
the figure, the weight given being the small 
column of mercury standing 29‘3 inches above 
the surface at (O) added to that of the tube ; 
then supposing that the pressure of the at- 
mosphere has increased so as to require the 
column to rise from 29*3 to 30*50 inches ; by 
tliis rise the equilibrium just effected will be 
destroyed, and the tube will descend. Sup- 
pose it continues to do so until the point 
marked 29*3 comes on a level with the sur- 
face at (0) ; at this time the enlarged part 
of the tube will be filled to 30-5 with a cy- 
linder of mercury, whilst the smaller part 
will have descended into the portion of the 
lower vessel made to receive it. Here, then, 
is given the weight, being that of the cylin- 
der of mercury in the enlarged part of the 
tube, 30*5 inches long, added to the weight of 
the tube, and the space through which it 
has descended (29*3 inches), to calculate the 
countervailing force at this terminating 
point. 
A twelfth of 29*3 inches will be, according 
to the supposition, the space through which 
the tube wall descend after the accession of 
each tenth of an inch to the head of the column ; 
and a twelfth of cylinder standing 30'5 inches 
in the enlarged part of the tube, will be the 
increase of weight to be counterbalanced after 
each such accession — the difierence of the 
columns (30*5 — 29*3= 1*2) containing twelve 
such accessions. 
Allowing the head of the column (l* 2 inch) 
to weigh, according to the diameter of the 
enlarged part of the tube, 29 ounces or pounds, 
when the tube has descended l inch, it will 
press on any obstacle, or the chain will turn the 
pulley, with a force of 28 ounces or pounds ; 
when it has descended 2 inches, it will act 
with a force equal to 27 ounces or pounds ; 
and so on decreasing one in effect for each 
inch it descends. It is easy, therefore, to 
assign the space through which the engine 
will exert any force within the premised limit. 
If, while the pressure of the atmosphere 
require that the column should rise from 29*3 
to 30*5 inches above the surface of the mer- 
cury at the foot of the tube, the syphon be 
removed, and the open end of the tube stop- 
ped for an instant, until the surface surroun- 
ding the foot can be depressed by any means, 
to a distance of 1*2 inch below (o), and then 
the stopping taken away, the condition (that 
the column shall stand 30*5 inches) being thus 
satisfied, no further rise will take place in 
the column. The accession of w^eight gained, 
in this case by the column, will be but that of 
a cylinder 1*2 inch long in the small end of 
the tube, and, consequently, the descent will 
be very trifling. 
Supposing the head of the column to rise 
exactly at the same rate that the surface below 
descends (as is the case in the tube of the j 
common wheel-barometer), the conditions^ 
imposed by atmospheric pressure, will be 
satisfied, when half as mueh mercury flows 
into the tube as would have risen into it had 
the surface at the foot remained stationary, 
and the power of the engine will be proportion- 
ally curtailed ; but the cylinder in the enlarged 
part of the tube, though thus shortened, will i 
be subject to change with the variations of the i 
column, in the same manner as that which 
has been described as filling the whole tube 
from 29*3 to 30i’5. 
Heretofore, to faeilitate description, the j 
cylinder of mercury in the enlarged part of 
the tube has been supposed to receive equal 
additions, at the times, the head of the co- 
lumn received equal additions to its height, i 
It is not, however, necessary that the addi- 
tions to the cylinder should be equal among I 
themselves, but only that the sum of the series " 
of them, added to the sum of the contemporary ' 
accessions to the head of the column, should 
together equal 30*5 inches ; or equal to the 
length of the longest cylinder in the enlarged 
part of the tube, due to the pressure of the i 
atmosphere and subject to the countervailing 
force. The engine is also capable of acting 
without the aid of a floating-cistern ; but cer- 
tain limits must be regarded, which it was the 
object of the foregoing remarks to notice. 
If the counterbalanee be effected by means 
of a weight andfuzee, the fuzee may be eut to 
the form of almost any spiral curve, and yet 
be within the requisite conditions ; still, how- 
ever, some of these will answer the purpose 
better than others. 
A self-registering barometer is easily con- 
structed on these principles ; for if the tube 
(which for this purpose should be, at least 
the upper part, of glass) be suspended imme- 
diately from a spring, or system of springs, 
the descent of which may be found by experi- i 
ment, and the points marked on a scale, the i 
tube being loaded for the purpose, a moveable j 
index may be made to show the greatest and 
least range of the tube within any given time, i 
The scale along which the tube will range ' 
will be equal in length to the greatest descent of • 
the tube, that is, in the foregoing example | 
29.3 inches. A couple of small weights sus- i 
pended on the face of the scale by fine silken 
threads passing over two delicate pulleys, and ' 
counterpoised, will mark the highest and low- ' 
est position of an index fixed to the tube ; 
and which may be made to raise one weight i 
up, or push the other down, as it passes ^ 
over the scale. If a floating-cistern be used, ! 
the scale of a common barometer may be also 
fixed at the head of the column ; this is a 
great advantage, and serves as a test of the i 
accuracy of the instrument. I 
The floating-cistern alluded to is provided ! 
to receive the mercury, or furnish it back i 
again, without raising or depressing the sur- | 
face of the mercury at the foot of the tube, i 
when the latter rises or descends, and, of 
course, empties or fills. It is only a vessel 
set on a stem, or float, which swims in an- 
