Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/399

Rh hundredth of an inch (O OIO), or even less, require to be obsarved and stated with great accuracy, the extreme importance of accurate sensitive barometers will be apparent, instruments not only possessing a great range of scale, but a scalo which will truly indicate the real atmospheric pressure at all times. The two barometers which best satisfy this requirement are King's Barometer, which has been in use for many years at the Liverpool Observatory, and Howson's Barometer. Fig. 4 shows the essential and peculiar parts of Howson s baro- meter. A is the barometer tube, which is of X large diameter, and longer than ordinary in order to admit of a greater length of range. B is a movable cylindrical cistern, having attached to its bottom 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, and its upper end is thus exposed to no more downward pressure than that of the mercury above it ; consequently, there is an excess of upward pressure of the air which tends to raise the cistern. When the ex cess of upward pressure is exactly balanced by the weight of the cistern with its stalk and con tained mercury up to 6, an equilibrium will be established, which will keep the apparatus sta tionary or hanging in suspension. If now the atmospheric pressure acting on the cistern be increased, and if the thickness of the glass tube A be supposed to be nothing, the cistern would continue to ascend to an indefinite extent, since there is nothing to stop it. But as the glass is a substance of some thickness, mercury is displaced by the glass as it is plunged further into the cis tern ; and as it thus otters a resistance to the ascent of the cistern, the cistern will come to rest when the quantity of mercury displaced is equivalent to the increase of pressure. The extent of range which this barometer possesses over the ordinary baro meter is determined by the ratio of the internal area of the tube A to that of the annulus of glass which bounds it, the range increasing as the internal area is increased, or as the thickness of the glass is diminished.

The liability of the barometer to bs broken in carriage is great. This risk is considerably lessened in the Board of Trade Barometer, which has the tube very much reduced in diameter for a part of its length, breakage from &quot;pumping&quot; being so much lessened thereby that the instrument may be sent as a parcel by rail, if only very ordinary care be taken in the carriage. This is essentially the principle of the Marine Barometer, which, however, has the tube still more contracted. For rougher modes of transit an ingeniously constructed iron barometer has been invented by Mr T. Stevenson, C.E.

The sympiezometer was invented by Adie of Edinburgh. It consists of a glass tube, with a small chamber at the top and an open cistern below. The upper part of the tube is filled with air, and the lower part and cistern with glycerine. When atmospheric pressure is increased, the air is compressed by the rising of the fluid ; but when it is diminished the fluid falls, and the contained air expands. To correct for the error arising from the increased pressure of the contained air when its temperature varies, a thermometer and sliding-scale are added, so that the instrument may be adjusted to the temperature at each observation. It is a sensitive instrument, and well suited for rough purposes at sea and for travelling, but not for exact observation. It has been for some time superseded by the Aneroid, which far exceeds it in handineas, portability, and correctness. The Aneroid Barometer was invented by Vidi, and patented in England in 1844. Its action depends on the effect produced by the pressure of the atmosphere on a circular metallic chamber partially exhausted of air and hermetically sealed. Fig. 5 represents the internal construction, as seen when the face is removed, but with the hand still attached. is a flat circular metallic box, having its upper and under surfaces corrugated in concentric circles. This box or chamber being partially exhausted of air, through the short tube b, which is sub sequently made air-tight by soldering, constitutes a spring, which is affected by every variation of pressure in the external atmosphere, the corrugations on its surface increas ing its elasticity. At the centre of the upper surface of the exhausted chamber there is a solid cylindrical projection x, to the top of which the principal lever cde is attached, as shown in the drawing. This lever rests partly on a spiral spring at d ; it is also supported by two vertical pins, with perfect freedom of motion. The end e of the lever is attached to a second or small lever/, from which a chain g extends to h, where it works on a drum attached to the axis of the hand, connected with a hair spring at h, changing the motion from vertical to horizontal, and regulating the hand, the attachments of which are made to the metallic plate i. The motion originates in the corru gated elastic box a, the surface of which is depressed or elevated as the weight of the atmosphere is increased or diminished, and this motion is communicated through the levers to the axis of the hand at h. The spiral spring on which the lever rests at d is intended to compensate for the effects of alterations of temperature. The actual movement at the centre of the exhausted box, from whence the indications emanate, is very slight, but by the action of the levers this is multiplied Go 7 times at the point of the hand, so that a movement of the 220th part of an inch in the box carries the point of the hand through three inches on the dial. The effect of this combination is to multiply the smallest degrees of atmospheric pressure, so as to render them sensible on the index. 5.—Aneroid Barometer. The instrument requires, however, to be repeatedly compared with a mercurial barometer, being liable to changes from the elasticity of the brass chamber changing, or from changes in the system of levers which work the pointer. Though aneroids are constructed showing great accuracy in their indications, yet none can lay any claim to the exact ness of mercurial barometers. The mechanism is liable to get fouled and otherwise go out of order, so that they may change O SOO inch in a few weeks, or even indicate pressure so inaccurately and so irregularly that no confidence can be placed in them for even a few days, if the means of com paring them with a mercurial barometer be not at hand.