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t. IX.] 



METEOROLOGY. 



9 Q ^, 



at tlie next 10% and in that case the entry in coliinin 4 

 must be not the whole amount of B - A, but only ftiis 

 of that amount, so as to reduce it to an interval of 60^ 

 precise. Column 5 contains readings of the internal 

 thermometer ; column 6 is left blank for the 



column 7 are entered re- 



^•'sult'" 



Ca 



o 



when reduced ; and in 



marks, such as the state of the sky, wind, &c. ; as also 



(when taken) the sun's altitude, barometer, thermometer, 



and other readings, &c. 



A complete actinometer observation cannot consist of 

 less than three sun and two shade observations inter- 

 mediate ; but five sun and four shade are m.uch better. 

 In a very clear sunny day it is highly desirable to 

 continue the alternate observations for a long time, even 

 from sunrise to sunset, so as to deduce by a graphical 

 projection the law of dhirnal increase and diniinution 

 of the solar radiation, which will thus readily become 

 apparent, provided the perfect clearness of the sky con- 

 tinue,— an indispensable condition in these observations, 

 the slightest cloud or haze over the sun being at once 



marked 



a 



diminution of resulting radiation 



To 



detect such haze or cirrus, a brown glass applied before 

 the eye is useful, and by the help of such a glass it may 

 here be noticed that solar halos are very frequently to be 

 seen when the glare of light is such as to allow no- 



+ 



thing of the sort to be perceived by the unguarded eye. 

 When a series is long continued in a good sun, the 



instrument grows very hot, and the rise of the liquid in 



the sun observation decreases, while the fall in the shade 

 increases ; nay, tow.irds sunset it will fall even in the sun. 



( 



t> 



