Memoir of M. D" Auhuisson de Voisins. 15 



that time, so many distinguished philosophers, Halley, 

 Bougner, Deluc, Laplace, Gay-Lussac, Ramond, Biot, and 

 Arago, have contributed to establish or bring to perfection. 

 In his sojourn at the foot of the Alps, M. D'Aubuisson found 

 a favourable opportunity of submitting this important matter 

 to the test of rigorous and enlightened experiment. In con- 

 cert with M. Mallet, chief engineer of Ponts-et-Chaussees, 

 now honorary inspector-general, he measured by ti'iangula- 

 tion (and with a precision which the most competent autho- 

 rities, MM. Laplace, Biot, and Arago, Commissioners of the 

 Institute, have acknowledged to be perfect) ; the height of 

 Mount Gregorie, a peak in the north of Piedmont, about 2000 

 metres above the sea, having its summit completely insula- 

 ted. He then measured the same height by means of the 

 barometer, with all the requisite precautions, on ten different 

 days ; and the application of his formula to this measurement, 

 gave him a mean height only two thousandths greater than 

 the trigonometrical method ; a very slight difference, but 

 which he availed himself of in order to correct the constant 

 coefficients of his two comparative formulae. Lastly, by ap- 

 plying the different known barometrical formulae to the same 

 measurements, he could submit them to a very interesting 

 comparative test. 



But this was not enough for M. D'Aubuisson ; he was de- 

 sii'ous that his abode among the Alps should enable him to 

 exhaust all that related to this important subject. He had 

 still to examine the horary and daily influence on the variable 

 cards of the barometrical method, to investigate the meaning 

 and limit of these errors, and the effect of each cause. For 

 this purpose, he went and set up a barometer at the hospice 

 of the Great St Bernard, the highest inhabited spot then 

 known, and for the space of fifty-two days he made conse- 

 cutive observations, either personally, undertaking frequent 

 and fatiguing journeys for the pui'pose, or by means of the 

 good monks who inhabited the hospice ; those observations 

 he compared, at the same time, with others made by a baro- 

 meter stationed' at Turin. This is not the place to enter into 

 a detail of these interesting experiments ; it may be merely 

 remarked, that the greatest influence was found to be that 



