Life and Writing^ of Theodore de Saussure. 11 



in the misierable hut on the Col du G^ant. During this jour- 

 ney, Theodore de Saussure endeavoured to verify, by a n6w 

 process, Bouguer's experiments on the density of air. Bou- 

 guer thought he had observed that at certain heights the 

 density of the air does not follow the ratio of the weights which 

 compress it. He estimated the resistance of the air, and con- 

 sequently its density, by the retardation in the movements of 

 a pendulum in a given space of time. Theodore de Saussure 

 had tried this mode of proceeding on the Col du Geant, and 

 believing it not to be very correct, he thought it necessary to 

 substitute another. This consisted in taking, at different 

 heights, the weight in the air of an empty glass ball of large 

 dimensions, and securely closed. It is evident that the differ- 

 ences in the weight of the ball would give exactly the differ- 

 ences in weight of the same volume of air which the ball dis- 

 placed at different heights. The capacity of the ball was 

 1053-95 cubic inches, and the balance by which it was weighed 

 yielded to the weight of half a grain, which was equivalent to 

 a thousandth part of the density of the air, under a pressure of 



27 (French) inches of the barometer. Theodore de Saussure made 

 70 experiments at heights varying from 1 8 inches 10 lines to 



28 inches of the barometrical column ; and he was careful, in 

 calculating the results obtained, to make the requisite cor- 

 rections for temperature and humidity, precautions which Bou- 

 guer appears to have neglected. He found, contrary to the 

 opinion of that natural philosopher, that the variations noticed 

 in the weight of the ball, or the density of the atmosphere, were 

 exactly proportionate to the pressures indicated by the baro- 

 meter. This investigation, the first, I believe, that Theodore de 

 Saussure published, was printed in the Journal de Physique 

 for February 1790, and restored the confidence, which the 

 assertions of Bouguer had tended to shake, in the measure- 

 ments of heights by the barometer. 



At Saint Marcel, while visiting the mines of that locality, so 

 celebrated among mineralogists, the travellers fell in with a cu- 

 rious fountain, called by the country people, the Blue Fountain. 

 The water is in sufficient quantity to drive a mill, and it is of 

 a deep sky-blue colour. Every thing is blue in its vicinity, 

 but the colour is paler when the soil is not moistened. Theo» 



