196 



On Solar Radiation. 



" Every series of actinometer observations should be accom- 

 panied with notices in the column of remarks, of the state of 

 the wind and sky generally, the approach of any cloud (as 

 seen in the coloured glass) near to the sun ; the barometer 

 and thermometers, dry and wet, should especially be read 

 off more than once during the series ; if a long one, and if 

 kept up during several hours, hourly. The times should 

 be correct to the nearest minute, at least as serving to cal- 

 culate the sun's altitude ; but if this be taken (to the nearest 

 minute or two) with a pocket sextant, or even by a style and 

 shadow frequently (at intervals of an hour or less) when the 

 sun is rising or setting, it will add much to the immediate in- 

 terest of the observations. When the sun is near the horizon, 

 its reflection from the sea or any neighbouring water must be 

 prevented from striking on the instrument ; and similarly of 

 snow in cold regions, or on great elevations in Alpine countries. 



" Every actinometer should be provided with a spare glass, 

 and all the glasses should be marked with a diamond ; and 

 it should always be noted at the head of the column of 

 remarks which glass is used, as the co-efficient of reduc- 

 tion from the parts of the scale (which are arbitrary) to parts 

 of the unit of radiation varies with the glass used. 



" In the case of the actinometers sent out with the Expedi- 

 tion and to the fixed Observatories, it was not practicable to 

 ascertain these co-efficients for each instrument and each 

 glass, owing to the total absence of any favourable opportu- 

 nity of sunshine. The values of the parts of the respective 

 scales of the instruments, as determined approximatively by 

 careful measurement of the dimensions, were as follows : — 



Marks of the Actinometers. 



Mark's mark. 



Private mark. 



1 



K 



2 



L 



3 



M 



A 4 



N 



A S 







6 



P 



Multiplier for reducing parts 

 observed to parts of a standard 

 retained in possession, marked 

 A. 1. 



Approximate value 

 of one part of scale 

 in Actines. 



1.4909 

 1.3726 

 1.4020 

 1.6550 

 1.4403 

 1.060S 



7.085 

 6.523 

 6.663 

 7.864 

 6.844 

 5.041 



