208 Instructions for the Scientific Expedition 



To use the instrument, examine first wlietlier tlicre l^e any air in 

 the cylinder, which is easily seen by holding it 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 thermometer-tube. Then, by alternate screwing and unscrew- 

 ing 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 up- 

 wards, slowly and cautiously unscrew the screw, till the liquid re- 

 treats 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 instru- 

 ment, liquid must first be forced up into the ball, so as to compress 

 the air in it. On warming the end, the wax Avill 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 

 acidulated, 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 tvith the actinometer, the observer must 

 station himself in the sunshine, or in some sharply terminated sha- 

 dow, so that without inconvenience, or materially altering his situ- 

 ation, 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 removeable 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 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, the better. 

 He should have a watch or chronometer beating at least twice in a 

 second, and provided with a second hand; also a pencil and 

 paper ruled, according to the form subjoined, for registering the ob- 

 sei-vations. Let him then grasp the instrument in his left hand, or 

 if he have a proper stand (which is preferable on shore or in a 

 building*), otherwise firmly support it, so as to expose its face per- 

 pendicularly to the direct rays of the sun, as exactly as may be. 



The liquid, as soon as exposed, will mount rapidly in the stem. 

 It should be allowed to do so for three or four minutes before the 



* This may consist of two deal boards, 18 inches long, connected by a hinge, 

 and kept at any required angle by an iron, pointed at each end. The upper should 

 have a httle rabbet or moulding fitting loosely round the actinometer, to prevent 

 its slipping off. 



