METEOROLOGY. 



161 



Meteorolo. for even- foot of elevation, and adding that quantity to 

 / the observed height. Like the former, this correction 



"~~ """"' is only an approximation to the truth, but on a subject 

 where there are so many other sources ot't';<ll;u-y . t-\trenie 

 accuracy in one point, is, in the language of an eminent 

 writer, an utter waste of the powers of calculation. U t - 

 hall now illustrate the application of these corrections 

 by an exantpV. 



Suppose the barometer is observed to stand at 29 565, 

 the attached thermoraefr being at the ame time 55, 

 or 2.1 above 33*. and the height of the place of obser- 

 vation 1.50 feet above the level of the sea, required the 

 altitude of die mercurial column, corrected a* above. 



Observed height 29-565 



Deduct for temperature, .003 X 23 . . .069 



Altitude if the mercury were at 32 . . 89.496 

 Add fur elevation of the place, .001 X 150 .150 



Altitude at the level of the e* . . . . 896(6 



Though these corrections are simple, and of easy ap- 

 plication, it is not likely that they will be generally 

 adopted, nor, if they were, could we expect to find a 

 very cloae coincidence between the results of different 

 barometrical observation*. The various degree* of de- 

 licacy in th instruments themselven, as well as of ac- 

 curacy in observing them, are sufficient to prevent a 

 constant harmony between two barometer*, even when 

 in the same circumstances ; and we have often 



BWUS toe- 

 """"of 



T*" 



frrrn'i 

 pUcn, by 

 Pictrt. 



, I. J 



and that in the case of very alight 

 ha* been elevated, while another appeared to be de- 

 pressed. At the same time, the observations that have 

 already been made, have established wcne interesting 

 facts regarding the simultaneous fluctuations in the 

 weight of the atmosphere at different, and even very 

 di-tint p! ii-i -. 



__ In a paper drawn up by Professor Pictet of Geneva, 

 on Ik* ni ' published in the second volume of Thomson's An- 

 nul*, we have a comparative view of the state of the ba- 

 rometer at London, Paris, ami Geneva, from S2d 8pt. 

 1806 toS.'-t Sept. 1807. The elevation* and depres- 

 sions of the mercurial column are represented by curve 

 line*, with the day r>f the month marked above or be- 

 low the extreme fluctuations, so as to show at one 

 glance both the time and the extent of these fluctua- 

 tion*. Were the change in the weight of the atmosphere 

 simultaneous and equal at the different place* where the 

 barometer was observed, and were the observation* 

 made at the came instant, and with the same degree of 

 accuracy, it is obvious that the curve line* should be 

 always parallel to one another. But though, in point 

 of fact, thit is found not to be the case, the deviation* 

 from parallelism are not greater than might be expect- 

 ed from the sources of difference and inaccuracy already 

 noticed. The extreme elevations and depression* ap- 

 pear to take place generally on the same day, and near- 

 ly Jo the same extent ; and it may be inferred, there- 

 thai changes in the weight of the atmosphere, at 

 London and Geneva, are almost simultaneous. Pictet 

 indeed ha* stated, that the fluctuation, when there i* 

 any difference in point of time, commonly takes place 

 at London sooner than at Geneva ; and he therefore 

 Lays it down as a general principle, that the disturbance 

 of atmospherical equilibrium travels from the north to- 

 ward* the south. This principle, however, has not 

 been verified by other and later observations. In a 

 paper by Lord Gray, inserted in the ,'<! volume of the 

 Mcmoin of the Wcrnerian Society, a comparative view 



XIV. PART I. 



i * given of the state of the barometer at Gordon Castle, Meteorolo. 

 latitude 57 38', Kinfauns Cattle 56 n 2V, Greenwich gy- 

 51S9', and Plymouth 50" 26' for 18I+, and at the ""-V""'' 

 first, second, and fourth of the above places for 181 5, 

 on the same plan as that adopted by Professor Pictet. 

 From this view, it appears that the fluctuations as be- 

 fore are nearly simultaneous; but when there is any 

 difference in point of time, the change commences in 

 the -.iuth just as frequently as in the north. We sus- 

 pect, indeed, that this apparent difference is in a great 

 measure owing to inaccuracy in observation. When 

 the barometer continues either to rise or fall for a con- 

 siderable time, its motion becomes slower a- it approach- 

 <- its extreme elevation or depression ; so that between 

 the evening of one day, and the morning of the next, 

 it may have risen or fallen only a few thousandths of 

 an inch. Unless, therefore, both observers are equally 

 accurate, one barometer may have appeared to rise and 

 the other to ff II, and consequently the extreme eleva- 

 tion or depression to have taken place on different 

 days. 



The same uncertainty, arising from the same causes, Periodical 

 attaches to many other observations that have been fluctuation* 

 msde. and conclusions that have been drawn, respect- of ' heb *ro- 

 ing the change* that take place in the pressure of the nulcl - 

 atmosphere. We allude particularly to certain perio- 

 dical fluctuations, which, it is alleged, have been de- 

 tected in the barometrical column, at different times of 

 the day, and in different positions of the moon. We do 

 not question the existence of certain causes operating 

 on the atmosphere, and producing at regular intervals 

 a change in its density ; but we apprehend, that amidst 

 so many other disturbing causes, and where observa- 

 tion* are so liable to error, we can know very little with 

 certainly rither of their nature or extent. Mr. Howard At 

 ha* stated, that on an average of ten years, he finds the 

 barometer is depressed one-tenth of an inch, while the 

 moon is passing from the quarters to the full and new ; 

 and elevated in the same proportion during the return 

 to the quarter. There can !>c no doubt that the com- 

 bined attraction of the sun and moon must operate more 

 powerfully than that of either of them singly, in dimi- 

 nishing the gravity of the atmosphere ; but whether the 

 quantity of this disturbing force is such as can be esti- 

 mated apart from others, is still, we think, very doubt- 

 ful. A daily periodical fluctuation of the barometer 

 ha* also been observed to take place within or near the 

 tropics ; but it does not appear to depend in any de- 

 gree on the state of the moon. From a meteorological At different 

 table kept at Seringapatam for 1816, and which we have houri of the 

 now before us, we find that the barometer almost uni- d>T> 

 formly stands higher there in the forenoon than in the 

 afternoon, at all seasons, and at every period of the 

 noon's age. From the regularity of this change, there 

 is no doubt that it depends on some fixed cause ; but 

 it seem* very difficult to account for it, and the more so, 

 as the temperature is generally highest while the baro- 

 meter is lowest. The multiplication of accurate regis- 

 ters in the tropical climates, where the range of the 

 mercury does not exceed half an inch, may perhaps 

 in time afford the mean* of solving this, as well aa 

 other interesting questions on the subject of atmosphe- 

 rical pressure. 



In our climate, the fluctuations of the barometer seem The baro. 

 to depend, in a great measure, on the direction and force meter chlf. 

 of the wind. In serene and settled weather, the mercu- '' <r c <d 

 ry generally stands about 30 inches at the level of the ** wind> 

 sea, and before or during storms it sinks below 29. The 

 range is commonly stated at 3 inches, viz. from 28 to 



iods of 

 the moon's 



S|C. 



