Sect, IX.] 



METEOROLOGY. 



289 



» 



Vi 



i 



% 



* 



i 



I - 



if 



I 



I ' 



1- 



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 be first 

 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 w^ashed 

 with water slightly acidulated, and the tube, ball, &c., 

 cleansed, in the same way, after which the wax must be 

 replaced, and the instrument refilled. 



To male an observation with the actinometer, the 

 observer 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 plea- 

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

 sun, he must provide himself wath a screen of pasteboard 

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

 part or chamber of the in 



instrument, which should be 

 placed not less than two feet from the instrument, and 

 should be removable 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 

 Sim shines freely. Draughts of air should be prevented 

 as much as possible. If the observations be made out of 

 doors, shelter from gusts of wind, and freedom from all 

 penumbral shadows, as of ropes, rigging, branches, &c.^ 

 should be sought. Generally, the more the observer is 

 at his ease, with his watch and writing-table beside him, 



o 



4£- 



*J^_ 







OS^^^ 



