190 On Solar Radiation. 



air should be prevented as much as possible. If the obser- 

 vations 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 pro- 

 vided with a second hand ; also a pencil and paper ruled, 

 according to the form subjoined, for registering the observa- 

 tions. 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 perpendicularly to the direct rays of the sun, as ex- 

 actly 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 observation begins, taking care, however, 

 not to let it mount into the bulb, by a proper use of the 

 screw. At the same time the tube should be carefully 

 cleared (by the same action) of all small broken portions of 

 liquid remaining in it, which should all be drawn down into 

 the bulb. When all is ready for observation, draw the liquid 

 down to zero of its scale gently and steadily ; place it on its 

 stand, with its screen before it, and proceed as follows : — 



" Having previously ascertained how many times (suppose 

 20) the watch beats in five seconds, let the screen be with- 

 draw at ten seconds before a complete minute shewn by the 

 watch, suppose at 2h. 14m. 50s. From 50s. to 55, say 0,0,0, 

 — at each beat of the watch, looking meanwhile that all 

 is right. At 55s. complete, count 0,1,2, up to 20 beats, 

 or to the whole minute, 2h. 15m. 0s. keeping the eye not 



* This may consist of two deal boards, eighteen inches long, con- 

 nected by a hinge, and kept at any required angle by an iron, pointed 

 at each end. The upper should have a little rabbet or moulding fitting 

 loosely round the actinometer, to prevent its slipping off. 



