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zontal lines on the ivory float and supports,
H. When the height of the barometer is required to be taken, the screw H is turned till the horizontal edge of the vernier forms a tangent to the convex curve of the mercury, as at fig. 3. Care must be taken to avoid a mistake not unfrequently made in shutting off the whole
Fig. 3. light; that is, instead of making the vernier a tangent to the mercurial curve, as it should be, it is made the arc of that curve.
36. A Travelling Barometer, fig. 4, has been constructed by Adie of London, which, since it possesses several advantages as a practical instrument, deserves to be noticed. The error of capillarity is allowed for in fixing the zero point of the scale. The error of capacity is obviated by making the inches of the scale not true inches, but just so much less as exactly counterbalances the error of capacity. The diameter
of the glass tube is generally contracted in its middle part, by which risk of breakage from "pumping" as it is conveyed from place to place is so much lessened that the instrument may be sent as a parcel by rail, if only the most ordinary care be taken in removing it in and out of the carriages. When the tube is very much narrowed through the greater part of its length, it forms what is commonly called the Marine Barometer. It is also frequently made with an air chamber, at A, fig. 2, by which any air that may accidentally find its way into the tube by the cistern is arrested in its ascent towards the top, and the instrument sustains no damage from the accident. Since this barometer may be conveyed from place to place with comparative safety, it is not liable to injury from the introduction of air into the Torricellian vacuum; and as it requires no preliminary adjustment of the cistern before making an observation, it is rapidly gaining favour with the public.
37. As the common cistern barometers are liable to be deranged by the introduction of air into their tubes on removal from place to place, or in being unskilfully handled, it is useful to know how the air may be expelled. First, close up the cistern, fig. 2, so as to prevent the escape of mercury, by fixing the ivory piston, F, which acts as a stopper to the cistern on being screwed up tight; then by the screw, P, raise the mercurial column to about half an inch from the top of the tube; and having slowly inverted the instrument, place the top of it against a yielding substance, such as the boot, and tap gently on the cistern with the palm of the hand, so as to induce the air to ascend to the cistern, whence it may escape. Since there is the weight of two atmospheres (the mercury in the barometer and the weight of the air outside) pressing on any air that may be inside the tube, it is usually a tedious operation to get it wholly ex
pelled. After repeated trials, however, it is generally accomplished; and the clear metallic click of the mercury, when struck against the top of the tube, by gently inclining the barometer, will show when the whole of the air has been got rid of. On hanging up the barometer, care must be taken to lower the mercury in the tube by turning the screw, P, before unfastening the float, F; for if this be not attended to, the pressure applied by the screw, P, will cause the mercury to flow out at the aperture of the float, thus seriously damaging the instrument.
38. In hanging or fixing the barometer, a perfectly perpendicular position must be secured; for if this be not attended to, the readings will be higher in proportion to the amount of the deviation from the perpendicular. Thus if, instead of hanging in a perpendicular line, it slant a little, it may read a tenth or more above what it ought to read. Hence, to obviate this risk of error, it is always preferable that barometers swing in position.
39. The Siphon Barometer, fig. 6, is made of a tube bent in the form of a siphon, and of the same diameter throughout. A graduated scale passes along the whole length of the tube, and when an observation is made, the readings at the lower and upper limbs are taken, and the difference between them is the height. There is another form of this instrument, in which, by means of a screw acting from below, the tube is made to slide along the scale, and hence in observing it is only necessary to set the surface of the mercury in the lower limb to the neutral point of the scale, and the pressure is seen at once from the upper limb. Since in siphon barometers the capillary attraction at both surfaces is the same, no error arises from this cause; and still further, there is no error of capacity from difference of level as in cistern barometers. But these advantages are counterbalanced by two disadvantages : 1, the trouble of taking two observations, and the chance of mistake Fig. 6.
which would frequently arise in taking the difference of the two; and, 2, the free contact of the open lower limb with the air, by which the mercury gets foul, and, adhering to the tube, impedes the smooth working of the instrument. In narrow tubes this becomes a very serious objection.
40. The common Wheel Barometer, the popular form of the weather-glass, is a modification of the siphon barometer. A small weight, glass or iron, floats on the mercury in the lower limb of the siphon: to this weight a thread is attached, which is led round a horizontal axis, a small weight being suspended at its free extremity to keep it tight. The glass float rises and falls with the fluctuations of the barometer, and a pointer fixed to the horizontal axis being turned by this means indicates the pressure by figures on a dial. Since the mercury only rises or falls in the open end of the siphon to the extent of half the oscillation, a cistern is added to the top to increase the amount of the variation in the lower limb. The lengthening and shortening of the thread with the dampness or dryness of the air, and the friction of the different parts, are causes of large and uncertain errors in this form of the barometer, and hence it cannot be used where accuracy is required. It is, however, useful in indicating roughly the more marked atmospheric fluctuations which are generally accompanied by changes of the weather.
41. It may be proper to refer here to what opticians have called the Fitzroy Barometer, which is a modified form of the siphon barometer. The lower limb is blown into a moderately sized bulb, so as to resemble a cistern. Since the price (20s.) is too low to admit of good tubes being used and the mercury boiled in them, and since the error of capacity is not got rid of, the instrument is of no use for scientific purposes. It is not unfrequently sold as a scientific instrument, to the annoyance of the buyer when he comes to understand its real value. It is useful as a weather indicator, in which respect it is a considerable improvement on the ordinary wheel barometer.
42. Houson's Barometer deserves to be noticed, not only for the great ability and ingenuity displayed in its invention, but because it may be justly considered as the best adapted for scientific purposes of that class of barometers which aim at great sensitiveness by increased range being given to their fluctuations. Fig. 7 shows the essential and peculiar parts of the instrument. A is the barometer tube, which is of large diameter, and longer than the ordinary sort, so as to admit of a greater length of range. B is a cylindrical cistern, having attached to the bottom of it a long hollow tube or stalk, c, hermetically sealed, springing to a height of about 28 inches above the fixed level of the mercury in the cistern. This stalk terminates a little below the upper level of the mercury, a, and its upper end is therefore exposed to no more downward pressure than that of the mercury above it; consequently there is an excess of upward pressure from the atmosphere which tends to raise the cistern. When the excess of upward pressure is exactly balanced by the weight of the cistern with its stalk and contained mercury up to b, an equilib- 8 rium will be established which will keep the cistern stationary or hanging in suspension. Suppose now that the atmospheric pressure acting on the cistern is increased, and that the thickness of the glass tube, A, immersed in the cistern, is nothing, the cistern will continue to ascend to an indefinite extent, there being nothing to stop it. The glass is, however, a substance of some thickness, displacing mercury as
Fig. 7. it is plunged into the cistern; and as it thus presents a resistance to the ascent of the cistern, the latter will come to rest when the quantity of mercury displaced is equivalent to the increase of pressure. The extent of range which this instrument possesses above the ordinary barometer is determined by the ratio of the internal area of the tube, A, to the annulus of glass which bounds it. If these two be equal the range is doubled, and if the internal area be greatly in excess the range is proportionately increased.