Donovan — On the Comparahle Self-registering Hygrometer. 167 



the unit by experiment. Thus, calculation and experiment agreed 

 exactly. 



Should it happen, through any cause, that ten revolutions of the 

 primary index do not measure the intercept in an artificial damp atmo- 

 sphere, a new revolution will commence. When the secondary indicates 

 10° in the open air (which it will very rarely do), we say that the air 

 is then satui'ated ; but the affinity of the gut-line for water may not 

 then be saturated. In an artificial damp atmosphere the gut-line will 

 still absorb, and continue to turn the indexes until the gut-line be 

 completely soaked, flaccid, and powerless". 



It has been shown in my former communication* to the Academy, 

 that a gut-string, as obtained from the music-seller, always contains 

 water. This, being not water of composition, but hygroscopic moisture, 

 may be abstracted without changing the nature of the animal matter 

 that contained it. It obeys the law of the instrument with regard to 

 absorption and expulsion. The indexes show how much had been 

 already absorbed, by previous exposure, which now entering undis- 

 tinguishably into present indications, keeps a running account of the 

 total water previously absorbed or newly acquired. 



* Vide "Proceedings of the Eoyal Irish Academy," vol. i., ser. ii., pp. 476 and 

 656. 



