328 BAROMETER cury when it is allowed to strike the top of the glass tube. Still the electrical light is supposed to be dependent on a small quantity of vapor left behind in the vacant space of the tube ; but in several instances it has been observed that the mercury remains suspended in the tube when this is inverted, even if the lower end be not placed in a cistern of the metal. It is de- tached by a sudden jar. The adherence of the mercury to the glass tends to introduce errors in estimating the true height of the column. Instead of forming at the top of the column a concave surface by the particles adhering to the glass and climbing up its surface, as water and other fluids do by the property called ca- pillarity, the mercury takes a convex form, and the column is lower than it should be. The smaller the bore of the tube, the greater is this depression and the error involved ; but in the siphon barometer (fig. 5) the error of one convex surface of the mercury in one limb is counteracted by the same effect from that of the other. However well constructed and filled, all barometers are liable to vary, after years of use, by a partial oxidation of the mer- cury, producing a thin film, which attaches it- self to and obscures the inner surface of the tube. This film can be removed only by clean- ing and refilling with fresh mercury. Air is liable to creep in between the mercury and the glass, and gradually enter into the vacuum, producing in the best instruments effects that are only perceived after a series of years ; in- struments used for a long period show a less height in the latter than in the former part of the period. Prof. Daniell constructed the most perfect water barometer ever made, which is somewhat similar to that already noticed of Guericke at Magdeburg. It is fixed in the hall of the royal society at Somerset house. The tube is of glass, 40 ft. long and an inch in diameter. The water in it stands at an average height of 400 inches above the fluid in the cistern. A layer of a solution of caoutchouc in naphtha upon the water in the cistern pre- vents access of any air to the tube. The column is sensitive to continual changes of pressure in the atmosphere, which do not affect other barometers. In windy weather it is in perpetual motion, vibrating up and down al- most with the regularity of respiration. It in- dicates the horary oscillations of the pressure sooner than does the mercurial barometer of half an inch bore. In the use of barometers, it is often desirable to have their variations recorded without the necessity of frequently observing them. Several methods have been devised of rendering them self-registering. One method is that of Mr. Bryson of Edin- burgh. Upon the mercury in the lower limb of a siphon barometer is placed an ivory float, which carries outside to the tube a knife edge. This, by proper machinery, is made to touch once every hour the surface of a vertical cyl- inder, which revolves with uniform motion once in 24 hours, and upon the face of which are marked spaces corresponding to the hours of the day and night. A new cylinder is used each day. The marks are made upon a coat- ing of fine chalk and water laid on with a camel's-hair brush. Such arrangements are, however, far inferior to the photographic method now adopted in all meteorological ob- servatories. This consists simply in a slip of sensitive photographic paper, moving by clock- work behind the upper part of the mercurial column, which throws its shadow on it, and thus prevents the impression of the light on the lower shaded portion. The light used is a kerosene lamp, and the slips of paper, after having been exposed, are darkened upon their upper half, while the undulating line be- tween the darkened and light portion shows the variations of the barometer during the time of exposure. Account should be taken of the temperature at the same time that the obser- vations of the barometer are noted; for the height of the column, as in the thermometer, must vary with change of temperature, as well as by change of atmospheric pressure. It is particularly important to make allowance for this cause of variation in observations for de- termining elevations, and a thermometer is al- ways attached to the barometer for this use. Between the points of boiling and freezing it is found that the space occupied by mercury amounts to jij of its bulk. For each degree of heat by the centesimal scale its volume in- creases TTTJ ! by Fahrenheit's thermometer, fTf-fj. Though little reliance can be placed upon the barometer as indicating by any single observation the condition of the weather, its fluctuations caused by changes of atmospheric pressure may, when care- fully noticed, often serve to foretell the effects that must still ensue. Thus, a sudden and long-continued fall is a sure sign of an impending storm. Many instances are recorded of vessels being saved by the precautions taken, in consequence of the warning of the barometer at the immediate approach of hurricanes, of which no other notice was given. Barometers have been con- structed with particular ref- erence to use at sea. (See fig. 8.) Their tube has a bore scarcely exceeding 3 V of an inch. Its upper end terminates in a cylinder 4 or 5 inches high and nearly T 3 <y of an inch in diameter. It is sus- pended by a spring and gimbals near the top. The object of the larger bore above the capil- lary tube is to prevent a rapid flow of the mercury, which might be caused by the motion of the ship, and break the tube by its striking against the top. The form is liable to the ob- jection that the rise and fall of the fluid is
