On Solar Radiation. 189 



level, and tilting it, when the air, if any, will be seen to run 

 along it. If there be any, hold it upright in the left hand, 

 and the air will ascend to the root of the thermometertube. 

 Then, alternately screwing and unscrewing the screw with 

 the right hand, as the case may require, it will always be 

 practicable to drive the air out of the cylinder into the ball, 

 and suck down liquid, if any, from the ball, to supply its 

 place, till the air is entirely evacuated from the cylinder, 

 and the latter, as well as the whole stem of the thermometer 

 tube, is full of the liquid in an unbroken column. Then, 

 holding it horizontally, face upwards, slowly and cautiously 

 unscrew the screw, till the liquid retreats to the zero of the 

 scale. 



" The upper bulb is drawn out into a fine tube, which is 

 stopped with wax. When it is needed to empty, cleanse, 

 and refill the instrument, liquid must first be forced up into 

 the ball, so as to compress the air in it. On warming the 

 end, the wax will be forced out, and the screw being then 

 totally unscrewed, and the liquid poured out, the interior 

 of the instrument may be washed with water slightly acidu- 

 lated, and the tube, ball, &c. cleansed, in the same way, after 

 which the wax must be replaced, and the instrument refilled. 



" To make an observation with the actinometer, the ob- 

 server must station himself in the sunshine, or in some 

 sharply terminated shadow, so that without inconvenience, 

 or materially altering his situation, or the exposure of the 

 instrument in other respects, he can hold it at pleasure 

 either in full sun or total shadow. If placed in the sun, 

 he must provide himself with a screen of pasteboard or tin 

 plate, large enough to shade the whole of the lower part or 

 chamber of the instrument, which should be placed not less 

 than two feet from the instrument, and should be remov- 

 able in an instant of time. The best station is a room with 

 closed doors, before an open window, or under an opening 

 in the roof into which the sun shines freely. Draughts of 



