162 



METEOROLOGY. 



Range of 

 the baro- 

 meter un- 

 der differ- 

 ent lati- 

 tudes. 



At diffe- 

 rent sea- 

 sons. 



31 inches ; but it appears doubtful whether an accu- 

 rate barometer on the sea shore will ever reach either of 

 these extremes. According to the most accurate ob- 

 servations, the average height seems to be 29-830, and 

 the range 2.5 inches. When the wind blows from the 

 north-east or east, the mercury generally stands above 

 the mean height, and below it when the wind is from 

 the south-west. To this, however, as well as every 

 other general maxim on the subject of the barometer, 

 there are numerous exceptions. The greatest elevation 

 of the mercury that we have observed during the last 

 six years was on the 30th November, 181(5, and it took 

 place with a west wind. On the morning of that day, 

 a barometer, 185 feet above the level of the sea, stood 

 at 30.640, and in the evening at 30.602, corresponding 

 respectively to 30.825, and 30.787 on the sea shore. 

 Sudden or violent gales of wind are generally preceded 

 or accompanied by a depression of the mercury, which 

 continues to fall as long as the gale increases, after 

 which it usually rises with as great rapidity as it fell. 

 The greatest depression of the barometer that has been 

 observed in this country for many years, took place on 

 the 5th of March, 1818, with a south-west wind. The 

 weather, for a considerable time before, had been stormy 

 and unsettled, with hurricanes of snow and rain from 

 the west. On the evening of the 4th the wind shifted 

 to the east, the barometer at the same time sinking ra- 

 pidly. During the night the wind got round again to 

 the south-west, and at eight on the morning of the 5th 

 the barometer stood at 27.970, corresponding to 28.155 

 at the level of the sea. On the same night the tide rose 

 unusually high in the river Tay, though it was only 

 the first of the spring tides, being two days prior to the 

 new moon ; and at the time that these phenomena were 

 observed in latitude 56 2V, a violent hurricane was ex- 

 perienced at London, and various other places in the 

 south of England. It frequently happens indeed, that 

 the barometer sinks during a gale of wind at a distance, 

 while the atmosphere at the place of observation is calm 

 and settled. We recollect having on one occasion, du- 

 ring close foggy weather, observed an unusual depres- 

 sion of the barometer unaccompanied with wind, but 

 afterwards found that it was contemporaneous with a 

 violent and disastrous storm on the coast of Norway. 



Many other facts might be stated, to show that the 

 fluctuations of the barometer are almost always propor- 

 tional to the strength and variableness of the wind. In 

 the tropical climates, where the latter is gentle and 

 steady, the range of the mercury is limited to a few 

 tenths of an inch, but increases rapidly from the tropic 

 towards the pole, where the winds are more violent and 

 irregular. In like manner, the elevations and depres- 

 sions of the mercury in the middle and higher latitudes, 

 are greater and more frequent in winter than in sum- 

 mer. We have found, on an average of three years, that 

 for the six months beginning with April and ending 

 with September, the mean monthly range of the baro- 

 meter, or the mean space between the highest and low- 

 est observation during the month, is to that of the other 

 six months in the proportion of 5 to 8. The same pro- 

 portion will also be found to hold between the two sea- 

 sons in the mean daily range. In our register for 1818, 

 we noted in separate columns the differences between 

 every two consecutive observations, which were made 

 at ten morning and evening, and extended to a third co- 

 lumn the sum of the two daily differences as the range 

 for the preceding 24 hours. The following is the re- 

 sult: 



Mean range for 24 hours, from April to Sept."? j- 4 Meteorolo- 

 inclusive, ..... J _ f y '_- 



Mean do. for the other six months, . .273 "~Y~" 



being nearly in the proportion of 5 to 8. 



It appears from the reports of Captain Christian, who indication! 

 carried out one of Adie's sympiesometers to the East of the sym- 

 Indies in 181 6 ; of Captain Ross and Lieutenant Robert- piesometer. 

 son, who employed one on board the Isabella discovery 

 ship ; and of Mr. Stevenson, engineer to the Scots light- 

 house board, that this instrument is still more easily af- 

 fected than the mercurial barometer by approaching 

 gales of wind. From a register kept by Lord Gray of 

 the contemporaneous indications of the sympiesometer 

 and barometer, as well as from observations which we 

 have had an opportunity of making with the same in- 

 strument, the former seems to be at least as delicate as 

 thelatter; but it is not quite apparent that the differences 

 between the two instruments always indicate a greater 

 degree of delicacy in the sympiesometer. We have now 

 before us a register of that instrument for one month, 

 with the corresponding indications of a very accurate 

 barometer taken twice a- day, the range of the mercury 

 during the period being from 29.190 to 30.380, and the 

 temperature from 54 to 64. Throughout the whole 

 of these observations, the sympiesometer stood higher 

 than the barometer, which might perhaps be account- 

 ed for from some inaccuracy in the construction of the 

 instrument ; but it is not so easy to explain the fact, that 

 this difference has been gradually on the increase. At 

 the commencement of our observations, it remained for 

 some days nearly stationary at .030, but during the last 

 eight days it has varied from .060 to .075. In the 

 course of Lord Gray's observations a similar increase 

 took place, which was afterwards however succeeded 

 by a diminution. It would be premature, therefore, to 

 decide on the merits of the instrument from such a li- 

 mited number of observations, though they naturally 

 lead to the suspicion that a certain degree of absorption 

 has taken place between the oil and the gas. But how- 

 ever this may be, there can be no doubt that the sym- 

 piesometer is, on some occasions, affected by changes 

 in the weight of the atmosphere, which are too minute 

 to make any sensible alteration in the mercurial column, 

 owing we conceive to the greater friction of the latter 

 on the internal surface of the tube. 



From these, and many other facts that might be pro- 

 duced, there can be no doubt of the relation between 

 the fluctuations of the barometer and the state of the 

 weather, particularly with regard to wind. It does 

 not follow, however, that the latter can in strict pro- 

 priety be assigned as the cause of the former. To say 

 that such a relation exists, is merely the statement of 

 two contemporaneous events, and still leaves the true 

 cause altogether unexplained. Wind itself is the effect 

 of some disturbance in the equilibrium of the atmo- 

 sphere, by which the barometer must necessarily be 

 affected, and it therefore remains to inquire by what 

 means that disturbance is produced. We shall resume 

 this subject, when we come to examine the phenomena 

 of wind and rain ; but before doing so, it will be ne- 

 cessary to consider the results of hygrometrical obser- 

 vations. 



We have already had occasion to notice the imperfec- Hrgrome- 

 tion of all the various hygrometers that were employed, trical ob- 

 previous to the invention of the thermoscope or differen- scration. 

 tial thermometer ; and we have still to lament that this 

 delicate instrument has hitherto been so little used by 

 meteorologists. It is also matter of regret, that in the 



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