Radiation . 399 
in Tasmania, at the expense of the British government, for the 
last five years. 
It is with these few remarks that the description of this instru¬ 
ment is introduced to the notice of the Society. Its name, the 
Actinometer, being derived from two Greek words, aktin , a 
beam of the sun, and ?nctreo, to measure: and its object is to 
ascertain, by direct experiment, the relative heating power of the 
sun’s rays. Hitherto, in the ordinary methods of estimating the 
sun’s calorific effect, the heating power of the sun’s rays, at the 
time of observation, has been put into equilibrium with the sur¬ 
rounding cooling influences of the atmospheric air, and other 
external causes, and the indications of an instrument (a thermo ¬ 
meter for instance), under such circumstances, taken as a mea¬ 
sure of the sun’s rays. Now, this is obviously faulty, because the 
temperature, maintained by a thermometer under such circum¬ 
stances, is as much a measure of the cooling , as of the heating in¬ 
fluences; and, in fact, is not a measure of either, because it is 
impossible to separate the effect of one from the other. The in¬ 
strument at present under consideration, proposes to overcome 
this very important objection, as the calorific rays penetrate and 
are absorbed at some sensible depth within its cylinder, the latter 
being filled with a deep blue liquid (ammonio-sulpliate of copper), 
selected from its chemical property of absorbing heat, and the 
liquid being heated within, the whole of the rays of the sun go to 
dilate the contents of the cylinder. Its component parts are, a 
large hollow glass cylinder, to which at its upper end is soldereda 
thermometer tube of great delicacy, attached to a divided scale of 
equal parts, the other end of the cylinder being closed by a silver 
plated cap cemented on it, and furnished with a silver screw pass¬ 
ing through a tight fitting collar, the action of the screw dimi¬ 
nishing or increasing, at pleasure, the capacity of the cylinder. 
The cylinder itself is enclosed in a blackened chamber on three 
sides, and the fourth, or face, protected from currents of air by a 
thick glass, removable at pleasure. As a very slight elevation of 
temperature is sufficient to expand the liquid through the whole 
column of the thermometer tube, the continued power of reading 
the indications on the scale is preserved, by withdrawing the screw 
