198 



On Solar Radiation. 



" The glasses as well as the cylinders and capillary stems 

 of the instruments, if accidentally broken, should have their 

 fragments carefully preserved and labelled. 



"The unit of solar radiation to be adopted in the ultimate 

 reduction of the actinometric observations is the actine, by 

 which is understood that intensity of solar radiation which 

 at a vertical incidence, and supposing it wholly absorbed, 

 would suffice to melt one-millionth part of a metre in thick- 

 ness, from the surface of a sheet of ice horizontally exposed 

 to its action per minute of mean solar time ; but it will be 

 well to reserve the reduction of the radiations as expressed 

 in parts of the scale to their values in terms of their unit, 

 until the final discussion of the observations. 



" Meanwhile, no opportunities should be lost of comparing 

 together the indications of different actinometers under simi- 

 lar and favourable circumstances, so as to establish a corres- 

 pondence of scales, which in case of accident happening to 

 one of the instruments, will preserve its registered observa- 

 tions from loss. 



" The comparison of two actinometers may be executed by 

 one observer, using alternately each of the two instruments, 

 thus : — 



Instrument. A. 



© 

 X 



© 



Instrument. B. 







X 



©• 



A. 



©■ 



X . 



© 



&c. 



beginning and ending with the same, though it would be 

 more conveniently done by two observers observing simulta- 

 neously at the same place, and each registering his own 

 instrument. An hour or two thus devot ed to comparison in 

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

 cases be extremely well bestowed. 



