VOL. LXXXI.] 



PHILOSOPHICAL TRANSACTIONS. 



115 



Table of the correspondent marches of slips, or of fibrous vegetable and animal substances taken across 

 the fibres, and of such as have no sensible fibres. 



This last table is the most important, as it contains a class of hygroscopes 

 which possess in common the following first requisites for an hygrometer ; 1st, 

 that they may indicate, without an illusion, both extreme dryness and extreme 

 moisture ; 2dly, that they move constantly in the same direction as moisture it- 

 self; 3dly, that they move always when moisture changes. It should seem as if the 

 march of the slip of horn taken lengthwise, from its very decreasing progression, 

 came very near that of the thin porcupine quill ; but, as Mr. D. has said, among 

 the steps of the latter there are accidental retrogradations, and it sometimes has a 

 final one ; and he has never observed that disposition in the former, which, in its 

 last small steps, follows constantly the motions of every other slip. 



The agreement of all the slips in this last respect is a very essential circum- 

 stance in hygrometry, as it assures us, that we cannot mistake the cases when 

 moisture is extreme in the atmosphere ; a very important point for discovering 

 the nature of many meteorological phenomena. No slip will create deception in 

 that respect ; while, on the contrary, every thread may deceive in dubious cases, 

 and even create great error, if, unknown to the observer, it happened to be in the 

 beginning of its elongation. There was however a question to be decided in that 

 respect, namely, whether or not a great moisture in the medium was a cause of 

 alteration in the march of any hygroscope, by producing in its substance a sudden 

 irregular lengthening. That accidental question is answered in the negative by 



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