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now of the International College, London, deserves special mention, in respect of his having, while mathematical master in the Church of Scotland's Normal School, Edinburgh, systematically trained the students to regular habits of meteorological observation. When the law of storms begins to be generally understood, and, as a consequence, the value of observations of the barometer, of the direction of the wind, and of the appearances of the clouds, comes to be appreciated as heralding changes of the weather, we cannot doubt that this very practical age will take steps to provide for the instruction of the people in the elementary facts of meteorology, and in the use of the different meteorological instruments.

CHAPTER II.

THE WEIGHT OR PRESSURE OF THE ATMOSPHERE.

27. The Barometer is the instrument employed to measure the height of a column of mercury supported by the pressure of the atmosphere. From this height the weight of the atmosphere is ascertained. The fundamental principle of the barometer cannot be better illustrated than by Torricelli's experiment. Take a glass tube (fig. 1), 33 inches in length, open at one end; fill it with mercury, and, closing the open end with the finger, invert it, and plunge the open end into a bowl (c) also containing mercury. The column will fall in the tube to about 30 inches above the surface of the mercury in the bowl, if the experiment be made near the level of the sea. The fluid is upheld in the tube by the air outside of it pressing on the mercury in the bowl ; and since the one thus balances the other, it is evident that the mercurial column will serve as an accurate indicator of the varying pressure of air. The space ab in the tube above the mercury is one of the nearest approaches to a vacuum that can be made. It is called the Fig. 1. Torricellian vacuum.

28. The heights of the columns of two fluids in equilibrium are inversely as their specific gravities; and as air is 10,784 times lighter than mercury, the height of the atmosphere

would be 10,784 times 30 inches, or nearly five miles, if it were composed of layers equally dense throughout. But since, from the diminished pressure and its great elasticity, air becomes less dense as we ascend, the real height is very much greater, being, it is supposed, about 210 miles.

29. Other fluids may be used in the construction of barometers, which, being lighter than mercury, have columns proportionally longer. Thus, if water, which is nearly 14 times lighter than mercury, be used, the barometric column is about 35 feet long. The advantage Water Barometers might be supposed to possess in showing changes of atmospheric pressure on a large scale, is more than counterbalanced by a serious objection. The space in the tube above the water is not a true vacuum, but is filled with aqueous vapour, which presses on the water with a force varying with the temperature. If the temperature be 32° the column is depressed half an inch, and if it be raised to 75o it is depressed a foot. In mercurial barometers the space in the top of the tube is no doubt filled with the vapour of mercury, but it exerts so slight a pressure that the amount of the depression, even at the ordinary temperature of a sitting-room, could not be measured by the most finely graduated vernier. Since no fluid approaches mercury in this respect, it is universally used in constructing the best barometers.

30. The tubes of barometers must be filled with pure mercury. If the mercury be not pure the density will be different, and consequently the length of the column will not be the same as that of a barometer in which pure mercury has been used; and, moreover, impurities will soon appear in the mercury, causing it to adhere to the tube, and, thus impeding its action as it rises and falls, will render the instrument useless for accurate observation.

31. In filling tubes, air and moisture get mixed up with the mercury, and, if not expelled, will soon ascend to the top of the tube, and, forming an atmosphere there, will depress the column. The air and moisture are expelled by boiling the mercury in the tube-always a difficult operation,

requiring the utmost precaution, so as to expel the air wholly without breaking the tube. As it is absolutely essential that a barometer be quite free from air and vapour, it should be tested some time after it has been boiled, and before it is used. This is done by gently inclining it, so that the mercury may strike against the top of the glass tube. If there be no air within, a sharp metallic click will be heard ; but if the sound be dull, the air must not have been completely expelled.

32. Barometers are commonly divided into two classescistern barometers and siphon barometers, the more important of these being the cistern barometer. Fig. 1 shows the Cistern Barometer in its essential and simplest form, and it only requires a scale, extending from the surface of the mercury in the cistern to the top of the tube, to make it complete. Cistern barometers are subject to two sorts of error, the one arising from capillarity, and the other from changes of level in the cistern as the mercury rises and falls in the tube.

33. The effect of capillarity is to depress the column, the amount of the depression depending on the internal diameter of the glass tube. Thus, if the diameter of the tube be half an inch, the error arising from capillarity is only .003 inch; if the diameter be į inch, the error is .012 inch ; if the diameter be inch, the error is .020 inch; and if the diameter be } inch, the narrowest tube that should ever be used, the error is .070 inch. Hence cistern barometers require an addition to be made to the observed height to give the true height.

34. The other error is technically called the error of capacity, and arises in this way: The height of the barometer is the distance between the surface of the mercury in the cistern and the upper surface of the mercury in the tube. Now suppose the barometer falls from 30 inches to 29 inches, an inch of mercury must flow out of the tube, and pass into the cistern, thus raising the level of the cistern ; if, on the other hand, it rises from 29 inches to 30 inches, mercury must flow back from the cistern into the tube, thus lowering the level of the cistern. Hence, then, owing to the incessant changes in the level of the cistern, the readings on the fixed scale are sometimes too high and sometimes too low. The simplest and rudest way of compensating for this error is to ascertain (1) the neutral point of the instrument—that is, the height at which it stands when the zero of the scale is on a level with the surface of the mercury in the cistern, or when it agrees with a standard barometer; and (2) the

rate of the error as the column rises or falls above this point, and to apply a correction proportioned to this rate. This is not only a clumsy method, but, as it involves some computation, it would give rise to frequent mistakes. The error is the less the more the surface of the cistern exceeds that of the column in the tube, because the fall or rise in the tube is spread over a larger surface. Hence it is desirable that the cisterns of barometers be made as large as possible.

35. The barometer in which the error of level is entirely got rid of is one invented by FORTIN; and since it is the best cistern barometer, we here describe it, or rather that modification of it in most general use in this country, figs. 2 and 5. In fig. 2, B is a brass box containing the cistern, C, the walls of which are of boxwood, but the bottom of flexible leather. This cistern contains the mercury, into which the glass tube is plunged. P is a screw, which works through the bottom of the brass box, B, against the flexible bottom of the cistern, by which the level of the mercury is raised or depressed at will.

F is a small ivory piston, to the foot of which O P is attached a float, which rests on the surface

of the cistern, and moves freely between the two ivory supports, I. There will also be observed hori

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