400 
On Solar 
and enlarging the capacity of the cylinder; and it is therefore 
evident, that employing time as one of the elements of the obser¬ 
vation, with a differential instrument of this kind, a comparative 
effect of the heating power of the sun’s rays, may be obtained in 
various parts of the globe, measured on an arbitrary scale of equal 
parts, which are afterwards convertible into the unit of solar ra¬ 
diation called the “ actinebut, in order to make these obser¬ 
vations comparative, it is essential that the same unit should be 
employed by all. In the ultimate reductions of the Actinometric 
observations, the unit (the “ actine”) will be understood, as that 
intensity of solar radiation which at a vertical incidence, and sup¬ 
posing it wholly absorbed, would suffice to melt one millionth 
part of a metre in thickness from the surface of a sheet of ice, 
horizontally exposed to its action, per minute of mean solar time ; 
and the determination of the co-efficient for reducing the obser¬ 
vations in parts of scale of the different instruments employed, to 
the actine, is ascertained, by measuring— 
1st, The external diameter of the cylinder containing the 
coloured liquid, which absorbs the rays. 
2nd, The length of that portion of it which receives the sun¬ 
beam. 
3rd, The content in water grains of 100 parts in length, of the 
capillary tube, used for the scale. 
The product of the two first, gives the area of the section of 
the sunbeam, effective in raising the temperature. Thus, in an 
instrument at the Observatory, the value of the scale, expressed 
in actines, is determined by a simple formula:— 
TV 
x = 2585.55 x- 
t* n . L.D 
where x = the number of actines corresponding to one division of scale 
2585.55 = a constant multiplier determined on, to reduce all the ob¬ 
servations, to a standard instrument retained in England. 
tv = 140 grains ; the weight of water displaced by 60 threads 
of the screw. 
t = 60.; number of turns of screw immersed in water. 
7i = 110.7 number of parts of scale half a revolution of the 
screw drives up the liquid. 
L = 7.2 inches, the length of the exposed portion of the cylin¬ 
der. 
D = 1.2 the external diameter of the cylinder. 
