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I 

 I 



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296 



METEOROLCGl 



i © 



[Sect, IX. 



tO£?ether the iridications of different actinometers imder 



similar 



and favonrable circumstancesy so as to establish 

 a correspondence of scales, which in case of accident 



happening to one of the instruments will preserve its 



registered observations from loss. The comparison of 

 two actinometers may be executed by one observer using 

 alternately each of the two instruments, beginning and 

 ending with the same ; though it would be more con« 

 veniently done by two observers observing simultaneously 

 at the same place, and each registering his own instru- 

 ment. An hour or two thus devoted to comparisons in a 

 calm clear day, and under easy circumstances, will in all 

 cases be extremely wdi bestowed. In frosty or very cold 

 weather the instrument should be exposed, for some time 

 previous to commencing tlie observations, to the sun, 

 which, by warming the liquid, increases its dilatability, 

 which at low temperatures is inconveniently small. 



Neither should each observer neglect to determine for 

 himself the heat stopped by each of his glasses. This 

 i^ay be done also by alternating quadruplet observations 

 made with the glass on and oiF, beginning and endin 



and (as in all cases) beginning and 

 ending each quadruplet with a sun observation. For the 

 purpose now in question a very ealm day must be chosen, 

 and a ereat many quadruplets must be taken in suc- 



with the glass oiF, 



cession 



The actinometer is w^cll calculated for measuring the 

 defalcation of heat during any considerable eclipse of 

 the sun. The observations should commence an hour 

 at least before the eclipse begins, and be continued an 



hour 



beyond its termination, and the series should be 



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