10 



PNEUMATICS. 



effect takes place when E falls and F 

 rises. It is evident that the scale on 

 which the index plays may, in this case, 

 be enlarged at pleasure. 



There are various other contrivances 

 for enlarging the scale of the barometer, 

 and other circumstances connected with 

 its construction, which we feel ourselves 

 precluded from entering on, by our ne- 

 cessary limits ; but the student who is 

 desirous of further information on the 

 subject, will find them treated of at large 

 in most works on mechanical science. 

 See Gregory" s Mechanics, vol. ii. 119 ; 

 Biot, Physique, torn. i. p. G9. 



(23.) As we ascend to greater heights 

 it is natural to expect that the atmos- 

 pheric pressure will be diminished, there 

 being a much less portion of atmosphere 

 above us ; and therefore the altitude of 

 the barometric column should be pro- 

 portionably lessened. And this we find 

 in fact to be the case. If a barometer 

 be carried to the top of high mountains, 

 or taken up in a balloon, the level of 

 the mercury in the tube will be observed 

 to fall as the elevation of the instrument 

 increases. At the level of the sea the 

 medium height of the barometric co- 

 lumn is twenty-eight inches, and at the 

 top of Mount St. Bernard it is only 

 fourteen inches. If the atmosphere re- 

 mained always in the same state, and, 

 like water, had at all heights the same 

 density, the barometer would, by the 

 property we have just mentioned, serve 

 as an accurate measure of the difference 

 of levels of two stations, or the differ- 

 ence of their perpendicularheights above 

 the level of the sea. For since the co- 

 lumn of mercury suspended in the tube 

 at each place is equal to the weight of 

 the column of atmosphere of the same 

 base, extending from that place to the 

 top of the atmosphere, it would follow 

 that the difference of the heights of the 

 columns (reduced to the same tempera- 

 ture (27.) ) would be equal to the weight 

 of a column of atmosphere, whose height 

 is equal to the difference of the levels of 

 the two places. If then the height of 

 a column of air, corresponding to that 

 of a column of one inch of mercury, be 

 known, it would only be necessary to 

 multiply this height by the number of 

 inches by which the barometric altitudes 

 at the two places differ, to obtain the 

 difference of levels. 



But the density of the air is not the 

 same at different heights. Air being 

 clastic, each inferior stratum suffers 

 compression from the incumbent weight 



of all the superior strata, and by this 

 compression its density is increased. As 

 we ascend in the atmosphere the quan- 

 tity of superior strata is gradually dimi- 

 nished, and the compressing force and 

 density proportionately diminished. This 

 change of density from level to level 

 renders the computation of heights by 

 the barometer somewhat more complex ; 

 but this would throw but little difficulty 

 in the way, if the density varied accord- 

 ing to some fixed and known law, and 

 * which would probably be the case if the 

 temperature of the air at all elevations 

 were the same. This, however, is not 

 the case. The temperature decreases as 

 the height of the station increases ; but 

 not regularly, nor according to any fixed 

 rule. The irregular variation in tem- 

 perature produces an irregular variation 

 in density, and therefore produces an 

 irregular variation in the change of the 

 barometric column. Notwithstanding 

 these irregularities, rules have been 

 determined, founded mainly on the prin- 

 ciple to which we have alluded, by which 

 the difference of levels of two places 

 may be computed, when the heights of 

 the barometer and thermometer "at the 

 two places are known. 



(24.) The changes in the altitude of 

 Ihe mercury in the barometer which we 

 have just been considering, have pro- 

 ceeded from the change in the altitude 

 of the barometer itself, with respect to 

 the earth's surface. But, besides this, 

 even when the place of the instrument 

 is not changed, when it remains sus- 

 pended in the same chamber, the surface 

 of the mercury, as we have stated, is 

 subject to rise and fall. This effect pro- 

 ceeds from a change in the state of the 

 atmosphere, and being continually ob- 

 served in connexion with the state of the 

 weather, it has been attempted to esta- 

 blish rules, by which changes of the 

 weather may be predicted from the va- 

 riations in the altitude of the barometric 

 column. Hence the barometer is also 

 called a weather-glass. It is proper, 

 however, to observe, that even the best 

 established rulesfor determining changes 

 of the weather by the barometer, are 

 very far from being either general or 

 certain. The rule which seems most 

 generally to obtain is, that the mercury 

 is low in high winds ; but even this fre- 

 quently fails. It is scarcely necessary 

 to observe, that the words rain, fair, 

 changeable, &c. engraved on the plates 

 -of common barometers, are entitled to 

 no attention. The changes of weather 



