295 HYGROLOGY, AND ITS CONNECTION WIT» METEOROLOGY. 



given degrees q^cintities of moisture, and the degrees of my hygrometer ; of 



ofthehygro- what pari each degree was of the whole: but I had not ob- 



mained to be" ^^i"^*^' a knowledge of the absolute quantity of evaporated water, 



found. which, in a giVen bulk of air, corresponded to these degrees ; a 



knowledge very essential in the investigation of the cause of 



rain. I saw that this was at least neccfs^ary for obtaining more 



certainty in meteorological conclusions. I relied in this respect 



on Mr. De Saussure's experiments, as I had net yet had time to 



undertake them myself j but I thought then to repeat the same 



experiments, for the following reasons, 



Mr, De Saus- ]3. Mr. De Saussure had made these experimeiits with his 



*^^^^^^^^l' hair-hysrovieter, which was so dissimilar to mine in the rate 

 ments objected , •^'- 



to. of lengthening with the same increases of moisture, that his 



results could not be immediately applied to my instrument. 

 But especially, he had made all these observations in the course 

 of one day j so that he could only obtain a tew immediate 

 points of comparison, whence he deduced a general law of the 

 correspondence of the degrees of his hygrometer with the 

 quantities oi evaporated water in a given bulk of air. This was 

 a first reason why some natural philosophers did not admit the 

 results <5f his experiments. There were also some other rea- 

 sons, which I shall hereafter mention : but these results were 



The author so important in meteorology, as he himself explained, that I 



nTatlhern* ^ ' ''^solved to repeat the same experiments in such a manner, as to 

 remove all the objections, which I clearly saw could only affect 

 the exactness of his experiments, but not their main results. I 

 shall now mention all these objections, and the mawner in 

 which I proposed to remove them. 



1st. objection. 14. The first objection, as I have said above, was the short 

 time employed in his experiments, to which he had been 

 obliged by the nature of his vessel : I therefore wanted to use 

 a vessel in which I could prolong these operations as long as I 



2nd. objection, should find it necessary, A second objection had been made 

 against the manner in which he first produced extreme dryness 

 in his vessel, which was by new-calcined salt of tartar ; a 

 substance which has chemical affinities with water, and might 

 absorb air with it : I wanted therefore to use new-calcined 

 lime, as I had used it for fixing the point q{ extreme dryness on 



ad. objection, my hygrometers. Lastly, there was an objection against the 

 manner by which he had determined the (juanlities of evaporated 



water 



