95 [ 237 ] 



the mean annual height of the barometer, and the mean annual tcinpera- 

 iure of the atmosphere, observed diirino; many years, at the same place. 

 With these two elements, we deduce the absohite ahitude of the place; 

 starling with the conventional principle, that at the level of the sea the mean 

 height of the French barometer is 0.7029 metre, and the mean temperature 

 12"^ S' centigrade; and of the English barometer, 30 inches, with a m.ean 

 temperature of 02° Fahrenheit. 



This method, because of its length, is not practicable for the traveller di- 

 rectly. Tile numerous observations which it demands belong, of course, to 

 a regular and permanent system of meteorological observations, having a 

 more extens•^ive object in view, and rarely met but in the public institutions 

 of large cities; so that, in fact, the traveller can only apply it as far as he 

 may have the good fortune to come, in his route, upon some station where 

 such a system has been established and followed with good instruments and 

 appropriate care. In such case, he avails himself of his good fortune to 

 have the absokUe height of the barometer at this station, and to refer to it 

 all the heights he may measure, until his distance from it no longer admits 

 his observations to be compared with those of the station. 



Nevertheless, he should not fail to institute a stationary system of meteor- 

 ological observations by himself, or by others, wherever he can find oppor- 

 tunity : independently of the immediate service that such observations ren- 

 der to meteorology, and a knowledge of the climate of the region, they be- 

 come, in their coniinuance, useful some day to himself or to others, for the 

 very method of which wc have been speaking, and for its varied and im- 

 portant applications. The extension of this method, from the long time it 

 takes, and the frequent observations it demands, will, doubtless, hereafter be 

 fertile in great results in this branch of physical science. 



In the philosophical study ot nature, where we seek to determine the laws 

 that govern progressive and variable phenomena, which are continually pre- 

 senting themselves, we want chiefly jmints of departure well fixed, and ob- 

 servations made with care, to show us the vicissitudes of the phenomena, so 

 as to connect the present and the past by numerical comparisons of deter- 

 minate epochs. In this regard, 1 may use the words of the Nestor of scien- 

 tific travellers. Baron Alexander Yon Humboldt : 



"If only in every thousand years the mean temperature of the atmo- 

 sphere and of the earth, in different latitudes, could have been determined, 

 or the mean height of the barometer at the shore of the ocean, we should 

 know in what ratio the heat of different climates has increased or dimin- 

 ished, and if any change has taken place in the height of the atmosphere." 



The second method is a compowid barometric levelUng^ and consists in 

 the compari^^on of the respective heights of the portable barometer with the 

 height of a stationary barometer, set up at a point whose absolute altitude is 

 known. Of course, the observations to be compared must be as much as 

 possible corresponding and simultaneous. If the station where the portable 

 barometer is observed be not very far distant from the otiier, which may be 

 called the fixed station, simultaneous readings of the two instruments, and 

 of their respective thermometers, every fifteen minutes, for several hours, 

 are enough to furnish tolerably exact data for the difference of level, espe- 

 cially if the observations are favored by calm weather, with the instruments 

 placed in the shade, and in isolated situations, so as to be exempt from ac- 

 cidental affection or local temperature. But the more remote the fixed and 



