16G Proceeding.^ of the Royal Irish Academy. 



XXIII. On SOIEE FtnETHER InPEOTDlIEXTS OF THE CoiEPAKABLE SeU- 



KEGISTEKLN'G HYGKOilETEH. By ^Jtl. DoXOTAJT, Esq. 



[Eead Februarj- 8, 187-5.] 



"Whatetee length of giit-line may be employed in giving one ronnd of 

 the piimary index, an equal length will be employed in giving every 

 other, the gut -line being equal thronghout ; since that length is the 

 natural unit, Tvhich, divided into the whole gut-line, gives for quotient 

 the number of rounds which the primary index, under the circum- 

 stances, is capable of accomplishing. The spiral convolutions of the 

 strands constituting the gut-line act simultaneously in all its parts, 

 but diminish in effect fi'om the bottom towards the top. Hence, the 

 shorter the intercepted portion of the gut-line is, the less of the 

 gTaduated circle will be traversed by either index in a given time, and 

 the greater will be the total number of rounds peiionned by the 

 primary. The revolutions of the primary index in the open air, 

 therefore, vii-tually divide the gut-line into as many parts as that 

 index performs revolutions round the graduated circle ; the intercept, 

 or what is the same thing, its effect in degrees on the graduated circle, 

 being the natural unit. 



According to the experiments of competent inquirers, the moisture 

 of the atmosphere and of pei'^'ious bodies can be completely absorbed 

 and withdrawn by exposure to the action of certain exsiccants, one of 

 the most efficient of which is chloride of calcium. With this substance 

 I proceeded as follows : — The hygrometer, fitted with a measured 

 length of gut -line, perfectly diied in the exsiccating receiver during 

 thi'ee days by means of ignited chloride of calcium, was placed in an 

 artificial damp atmosphere, and so left for 24 hour's, during which time 

 the primary index had moved very nearly nine times round, and had 

 begun to move backwards. A slip of wet blotting-paper being intro- 

 duced into the receiver, the index continued to recede, some aii- having 

 entered ; but after a short while advanced, and at length reached zero, 

 thus completing ten roujids, shortly after which it permanently ceased 

 to move. Thus, in the open air, ten rounds of the primary index, or 

 one of the secondary, cannot be exceeded, for they comprise all the 

 degrees between extreme moistui'e and extreme dryness. The length 

 of the gut-line exposed to the action of the exsiccant being 4'65 laches, 

 and that of the intercept "6 inch, the resulting normal number of 

 calculated rounds by the primary index would be 7"75 ; and the nm' t, 

 in the open air, at this rate, would be by calculation 12°'9. But the 

 experiment having been repeated in an artificial saturated damp 

 atmosphere, the gut-line untwisted to ten rounds of the primaiy index, 

 which, therefore, gave 10° as the unit; and the average of the differ- 

 ences in 10 rounds (-siz., 10, 10, 9, 10. 11, 8, 12, 10. 10, 10.) gave 10° as 



