250 071 the variable Action of the Electric Column^ 



circumstance I continue to think must be owing to the difterencrf 

 in the electrical state of the amli'dut air ; and thus the column 

 is, as I had found it in my first observations, an aerial electrO' 

 scope, which property Mr. Ronalds had also surmised. But too 

 many causes interfere with that state of the ambient air, to di- 

 stinguish that effect with certainty ; and it will require a longer 

 study of the variations of the effects by different circumstances, 

 in order to assign the true influence of each of them. 



Mr. Ronalds's experiment 7 is a confirmation of those which 

 I had described in a paper published in the Philosophical Trans- 

 actions of the Royal Society for 1791. These experiments 

 were made in order to show the cause of a dissentiment between 

 ^I. de Saussure and myself on the })oint of extreme moislnre, 

 which I had prescribed to be taken in water itself; the reason 

 of which I stated in my first paper on the Hygrovielcr, published 

 in the Philosophical Transactions for 1774. The construction 

 of M. de Saussure's hygrometer did not permit to dip it in 

 water, because the index Avas in the lower part of the frame, 

 and the hair of which it is formed, could not l)e dipped in i^a/fcr 

 without the whole instrument, its index and scale, being im- 

 mersed into water: he therefore fixed its point ci extreme mois- 

 ture under a receiver inverted on a dish full of water. 



Unluckily this circumstance did not permit M. de Saussure to 

 discover the defect of the hair and of all the threads oi' Jihrous 

 substances, which is to relent successively their lengthening when 

 moisture increases, and even at last to grow shorter, more or less 

 according to their nature, when inoisiiiie approaches to its ex- 

 treme ; which circumstance excludes all the threads from a true 

 hYgrometer. But my hygrometer consisting of a slip of whaler 

 lojie cut across the fibres, is not subject to that defect ; it lengthens 

 up to the point of extreme moisture taken in ivater, and witli 

 it I made the same experiment under the receiver inverted on 

 water ; and thus 1 found the important law of hygrology, that 

 the utmost quantity of evaporated water under such a receiver 

 does not produce extreme moisti/re in the inclosed space, when 

 the degree of heat is sensibly above 32 ; and that it recedes 

 from it in proportion as the heat increases. Mr. Ronalds has 

 found the same effect in his experiment 7 ; for, as long as the 

 temperature was 33, which lasted a long while, the hygrometer 

 never attained a point higher than 93. 



In the same experiment under the receiver inverted over water, 

 M.Ronalds found the 7inniber ot slrikings gniduaWy to decrease, 

 from 4*5 in a minute even to no striking. 1 am not surprised at 

 this effect, as it proceeds from the cause which I have already 

 indicated ; for the glass tube lu which the column was inclosed 



bl?ing 



