BAROGRAPHS, OR SELF-REGISTERING BAROMETERS.
=33. Milne's Self-Registering Barometer.=--For a long time a good and
accurate self-recording barometer was much desired. This want is now
satisfactorily supplied, not by one, but by several descriptions of
apparatus. The one first to be described was the design of Admiral Sir A.
Milne, who himself constructed, in 1857, we believe, the original
instrument, which he used with much success. Since that time several of
these instruments have been made, and have performed satisfactorily. The
barometer tube is a syphon of large calibre, provided with a Gay Lussac
pipette, or air-trap; and fitted with a float, a wheel, and a pointer, as
in the "Dial" barometer. The float is attached to a delicate watch-chain,
which passes over the wheel and is adequately counterpoised. Behind the
indicating extremity of the pointer or hand is a projecting point, which
faces the frame of the instrument, and is just within contact with the
registering paper. A clock is applied, and fitted with auxiliary
mechanism, so as to be able to move the mounted paper with regularity
behind the pointer, and at designed equal intervals of time to release a
system of levers and springs, so as to cause the marker to impress a dot
on the paper, either by puncture or pencil-mark. The paper is ruled with
horizontal lines for the range of the mercurial column, and parallel arcs
of circles for the hours. Thus the barometer is rendered self-recording,
by night or day, for a week or more; hence the great value of the
instrument. The clock, index, and registering mechanism are protected from
dust and interference by a glass front, hinged on and locked. As the
temperature of the mercury is not registered, there is fixed to the frame
a Sixe's thermometer to record the maximum and minimum temperatures, which
should be noted at least every twenty-four hours.
Admiral FitzRoy has suggested the name "Atmoscope" for Admiral Milne's
barometer; and he has also termed it a "Barograph." This latter word
appears to be applicable to all kinds of self-registering barometers
hitherto designed. Of the arrangement under consideration Admiral FitzRoy
writes:--"It shows the alterations in tension, or the pulsations, so to
speak, of atmosphere, on a large scale, by hourly marks; and the diagram
expresses, to a practised observer, what the 'indicator card' of a
steam-cylinder shows to a skilful engineer, or a stethescope to a
physician."
[Illustration: Fig. 26.]
=34. Modification of Milne's Barometer.=--The great difficulty to be
overcome in Milne's barometer, is to adjust the mechanism for obtaining
registration so that the action of the striker upon the pointer should not
in the slightest degree move it from its true position. A different mode
of registration, capable of recording accurately the least appreciable
movement of the mercurial column, has been effected. In this instrument
the registering paper is carried upon a cylinder or drum. By reference to
the illustration, Fig. 26, the details of construction will be readily
understood. It should, however, be mentioned, that it is not a picture of
the outward appearance of the instrument. The position of the barometer
should be behind the clock; it is represented on one side merely for the
purpose of clearly illustrating the arrangement and principles. The
instrument has a large syphon barometer tube, in which the mercurial
column is represented. On the mercury at _A_, in its open end, rests a
glass float, attached to a watch-chain, or suitable silken cord, the other
end of which is connected to the top of the arched head on the short arm
of a lever-beam. The long arm of the beam is twice the length of the short
arm, for the following reason. As the mercury falls in the long limb, it
rises through an equal space in the short limb of the tube, and _vice
versa_. But the barometric column is the difference of height of the
mercury in the two limbs; hence the rise or fall of the float through
half-an-inch will correspond to a decrease or an increase of the
barometric column of one inch. In order, then, to record the movements of
the barometric column, and not those of the float, the arm of the beam
connected with the float is only half the radius of the other arm. Both
arms of the beam carry circular-arched heads, which are similar portions
of the complete circles, the centre of curvature being the fulcrum, or
axis. This contrivance maintains the leverage on each extremity of the
beam always at the same distance from the fulcrum. From the top of the
large arched head a piece of watch-chain descends, and is attached to the
marker, _B_, which properly counterpoises the float, _A_, and is capable
of easy movement along a groove in a brass bar, so as to indicate the
barometric height on an ivory scale, _C_, fixed on the same vertical
framing. On the opposite side of the marker, _B_, is formed a metallic
point, which faces the registration sheet and is nearly in contact with
it. The framing, which carries the scale and marker, is an arrangement of
brass bars, delicately adjusted and controlled by springs, so as to permit
of a quick horizontal motion, in a small arc, being communicated to it by
the action of the hammer, _E_, of the clock, whereby the point of the
marker is caused to impress a dot upon the paper. The same clock gives
rotation to the hollow wooden cylinder, _D_, upon which is mounted the
registering paper. The clock must be rewound when a fresh paper is
attached to the cylinder, which may be daily, weekly, or monthly,
according to construction; and the series of dots impressed upon the paper
shows the height of the barometric column every hour by day and night. The
space traversed by the marker is precisely equal to the range of the
barometric column.
=35. King's Self-Registering Barometer.=--Mr. Alfred King, Engineer of the
Liverpool Gas-light Company, designed, so long ago as 1854, a barometer to
register, by a continuous pencil-tracing, the variations in the weight of
the atmosphere; and a highly-satisfactory self-recording barometer, on his
principle and constructed under his immediate superintendence, has quite
recently been erected at the Liverpool Observatory.
[Illustration: Fig. 27.]
Fig. 27 is the front elevation of this instrument. _A_, the barometer
tube, is three inches in internal diameter, and it floats freely (not
being fixed as usual) in the fixed cistern, _B_, guided by
friction-wheels, _W_. The top end of the tube is fastened to a peculiar
chain, which passes over a grooved wheel turning on finely-adjusted
friction rollers. The other end of the chain supports the frame, _D_,
which carries the tracing pencil. The frame is suitably weighted and
guided, and faces the cylinder, _C_, around which the tracing paper is
wrapped, and which rotates once in twenty-four hours by the movement of a
clock. Mr. Hartnup, Director of the Liverpool Observatory, in his Annual
Report, 1868, says:--"For one inch change in the mercurial column the
pencil is moved through five inches, so that the horizontal lines on the
tracing, which are half an inch apart, represent one-tenth of an inch
change in the barometer. The vertical lines are hour lines, and being
nearly three-quarters of an inch apart, it will be seen that the smallest
appreciable change in the barometer, and the time of its occurrence, are
recorded."
"It has been remarked by persons in the habit of reading barometers with
large tubes, that, in squally weather, sudden and frequent oscillations
of the mercurial column are sometimes seen. Now, to register these small
oscillations must be a very delicate test of the sensitiveness of a
self-registering barometer, as the time occupied by the rise and fall of
the mercury in the tube in some cases does not exceed one minute." Mr.
Hartnup affirms that the tracing of this instrument exhibits such
oscillations whenever the wind blows strong and in squalls.
As the barometer in this instrument is precisely similar to the "Long
Range Barometer" invented by Mr. McNeild (and which will be found
described at page 48), it may be desirable to quote the following, from
Mr. Hartnup's Report:--"Mr. King constructed a small model instrument to
illustrate the principle. This instrument was entrusted to my care for
examination, and it was exhibited to the scientific gentlemen who visited
the Observatory in 1854, during the meeting of the British Association for
the Advancement of Science."
=36. Syphon, with Photographic Registration.=--A continuous
self-registering barometer has been constructed, in which photography is
employed. Those who may wish to adopt a similar apparatus, or thoroughly
to understand the arrangements and mode of observation, should consult the
detailed description given in the _Greenwich Magnetical and Meteorological
Observations_, 1847. As the principles are applicable to photographic
registration of magnetic and electric as well as meteorologic variations
in instrumental indications, it would be beside our purpose to describe
fully the apparatus.
The barometer is a large syphon tube; the bore of the upper and lower
extremities, through which the surfaces of the mercury rise and fall, is
1-1/10 inch in diameter. The glass float in the open limb is attached to a
wire, which moves a delicately-supported light lever as it alters its
elevation. The fulcrum of the lever is on one side of the wire; the
extremity on the other side, at four times this distance from the fulcrum,
carries a vertical plate of opaque mica, having a small aperture. Through
this hole the light of a gas-jet shines upon photographic paper wrapped
round a cylinder placed vertically, and moved round its axis by a clock
fixed with its face horizontal. The cylinder is delicately supported, and
revolves in friction rollers. A bent wire on the axis is embraced by a
prong on the hour hand of the time-piece; therefore the cylinder is
carried round once in twelve hours. It might be arranged for a different
period of rotation.
As the cylinder rotates, the paper receives the action of the light, and a
photographic trace is left of the movements of the barometer four times
the extent of the oscillations of the float, or twice the length of the
variations in the barometric column. Certain chemical processes are
required in the preparation of the paper, and in developing the trace. The
diagram which we give on the next page, with the explanation, taken from
Drew's _Practical Meteorology_, will enable the above description to be
better understood:
[Illustration: Fig. 28.]
"_Q e_ is a lever whose fulcrum is _e_, the counterpoise _f_ nearly
supporting it; _s_ is an opaque plate of mica, with a small aperture at
_p_, through which the light passes, having before been refracted by a
cylindrical lens into a long ray, the portion only of which opposite the
aperture _p_ impinges on the paper; _d_ is a wire supported by a float on
the surface of the mercury; _G H_ is the barometer; _p_, the vertical
cylinder charged with photographic paper; _r_, the photographic trace;
_I_, the timepiece, carrying round the cylinder by the projecting arm _t_.
It is evident that the respective distances of the float and the aperture
_p_ from the fulcrum may be regulated so that the rise and fall of the
float may be multiplied to any extent required." When _only_ the lower
surface of the mercury in a syphon barometer is read, as in the instrument
just described, a correction for temperature is strictly due to the height
of the quicksilver in the _short_ tube; but this in so short a column will
rarely be sensible.