346 



THE POPULAR EDUCATOR. 



PNEUMATICS. IV. 



THE BAROMETER (continued) HOW USED TO FORETELL 



CHANGES IN THE WEATHER SELF- REGISTERING BAROMETER 

 HOW USED TO MEASURE HEIGHTS. 



MANY very skilful arrangements have been made to guard 

 against the errors referred to in our previous lesson ; and a 

 barometer was constructed some time since by a gentleman in 

 Birmingham, which embraced nearly all these. The surface of 

 the water in the reservoir was covered with oil to tho depth of 

 an inch or two, so as completely to exclude 

 the air ; the upper part of the tube was pro- 

 longed into a spiral coil, which could be 

 cooled so as to condense the vapour ; the 

 utmost care was also taken in filling the 

 tube. It was then found to be very much 

 more sensitive than the common mercurial 

 barometer. During a storm, while the latter 

 only showed a slight variation, this showed 

 extensive oscillations rapidly succeeding each 

 other. It was found, too, that changes in 

 the air were shown by this a full hour sooner 

 30 I than by the ordinary instruments ; but, de- 



spite these facts, the common barometer is 

 the more to be depended on, in the long run, 

 as the mercury only evaporates in a very 

 slight degree, and is easily obtained per- 

 fectly pure. By means, too, of a sliding 

 scale called a vernier (Fig. 11), the height 

 can with ease be read to within rta of an 

 inch, and this is sufficient for most purposes. 

 The vernier consists of a pointer attached to 

 a scale, which can be moved up and down so 

 as to adjust it exactly to the level of the 

 mercury. The ordinary scale is divided into 

 inches and tenths of an inch ; the vernier, 

 however, is exactly It's inches long, and is 

 divided into ten equal parts. Each division 

 is therefore ^njth of an inch. If any division 

 of this be made to correspond with one on 

 the ordinary scale, the adjacent divisions on 

 each will be rJs of an inch apart, the next 

 T$^, and so on. The divisions on it are 

 Fig. 11. marked downward, 1 being at the top. Now 



we will suppose the mercury to stand as 

 shown in the figure ; the pointer is adjusted to the level, and 

 we immediately see that the height is somewhere between 29'8 

 and 29'9. We now run our eye down the vernier till we find 

 the division which is most nearly even with one in the 

 A other scale ; this is the one marked 6. The level is there- 

 fore rStj above 29'8, or 29'86. 



In a barometer the mercury always clings, to a certain 

 extent, to the side of the tube, and thus seldom presents 

 an even surface ; the readings ought, therefore, to bo taken 

 from the height of the centre of the column. There is 

 one great advantage derived from this, viz., that we can 

 see at a glance whether it is rising or falling. If it is 

 rising, the surface is convex, or higher in the middle, that 

 at the sides being kept back by its adhesion 

 to the tube ; while if it is falling, the surface 

 is concave. 



The cistern barometer which we have been 

 considering is the more common form of the 

 instrument. The wheel barometer is, how- 

 ever, often used, and we must therefore give 

 a description of it (Fig. 12). It consists of 

 a large dial-plate fixed near the lower end of 

 an oblong case, and round it are graduations 

 from 28 to 31 inches, and also the words 

 Stormy, Much Bain, Bain, Change, Fine, 

 Set-fair, and Very Dry. A hand, turning on 

 an axle, points to different parts of the face, 

 and thus gives the readings. If we open the 

 case behind, we shall see that the main 

 difference is that the end, instead of opening 

 into a cistern, is turned up to a height of 6 

 or 7 inches, and a float G rests upon the surface of the mer- 

 cury in this limb. This float is attached to a cord, which 



Fig. 12. 



passes over the wheel H, and has a small counterpoise, w 

 fastened to the other end. The hand seen on the dial-plate is 

 attached to this wheel. When tiiC mercury falls in the limb 

 A B, it rises in the shorter limb B c to an equal eiisr.t; the float 

 is therefore raised, and the weight w turns the hand, which 

 thus shows the height. On tho mercury falling again, tha 

 weight of Q more than balances w, and brings the hand back 

 again. In this form of barometer the surface of the mercury 

 cannot be seen so as to tell whether it is rising or falling ; an 

 additional hand, worked by a small handle below, is, however, 

 placed on the dial, and registers the position at any time, and 

 thus shows at once whether it has risen or fallen since it was 

 last set. 



The upturned end of the barometer is sometimes enlarged, 

 and a stop-cock inserted just under the enlargement, so that by 

 inclining the tube it becomes filled up to the top ; the tap may 

 then be closed, and the superfluous mercury poured away. In 

 this way it may be carried about with safety, as the tube 

 is completely filled, and all vibration thus prevented. On 

 turning the tap, and placing the tube in a vertical position, the 

 pressure will at once be shown. Care must, however, be 

 taken that the tube is vertical, as otherwise the mercury 

 will appear to stand at a greater height in the tube than it 

 really does. 



A brief allusion has been made to the " water barometer," 

 by which the variations of the weather may be more readily 

 detected, but which is inconvenient on account of its great 

 length, and consequent unwieldiness. Among curious construc- 

 tions on this principle may be mentioned the " water barometer " 

 of Otto Guericke, which was attached to a wall with a toy in the 

 form of a man floating on the water. The entire tube was 

 hidden behind some wainscoting, so that the little figure was 

 only seen, appearing and disappearing, as the weather was fine 

 or the reverse. 



Having seen the mode of construction of the barometer, and 

 the precautions which are taken to en-ure its accurate reading, 

 we must now see the way in which it is employed to foretell 

 changes in the weather, or to show the elevations of different 

 places. 



The barometer itself simply informs us what is the pressure 

 exerted by the air at any time and place. This pressure is 

 found to exhibit very great variations from time to time, and 

 one of the causes of these is the moisture or dryness of the air. 

 Moist air that is, air charged with watery vapour is found to 

 weigh less than an equal bulk of dry air. This seems strange, 

 but the barometer shows us that it is so ; and we have a further 

 proof of the fact if we observe the smoke from a chimney. On 

 a fine day the air is heavy, and buoys it up in an almost straight 

 line, while on a damp day it falls heavily. When, therefore, the 

 air is charged with vapour, its pressure diminishes, and the 

 barometer therefore falls. We have thus this general rule : 

 When the barometer is low, wet or windy weather may be ex- 

 pected ; and, on the contrary, when it is high, the weather will 

 not improbably be fine. 



This rule, however, is very vague ; and if taken by itself will 

 often mislead. The words "much rain," "rain," "change," 

 "fair," etc., usually inscribed on the dial of a wheel barometer, 

 are also almost useless, as no correct inferences can be drawn 

 from them, the actnal height at which the barometer stands 

 being a far less certain guide than the fact of the column being 

 in a rising or falling state, as shown by the convexity or con- 

 cavity of its surface. 



There are three things which mainly affect the height of the 

 mercury. These are 



1. The force of the wind, which produces variations occasion- 

 ally amounting to as much as two inches. 



2. The amount of moisture in the air. The variations from 

 this cause amount to about half an inch. 



3. The direction of the wind ; a north-east wind having a 

 tendency to cause a rise, and a south-west wind a fall. Theso 

 variations likewise amount to about half an inch. 



From this it will be seen that wind affects the barometer 

 much more than moisture does. It is important, then, to notice, 

 together with the height of the barometer, the direction of the 

 wind, and likewise the temperature at the time of taking tho 

 observation. 



Space will not allow us here to give full instructions as to in- 

 terpreting the barometer. We subjoin, however, a few general 



