194 On Solar Radiation. 



35.30 = + + 33.4, &c. 



2 

 " The mean of a series not exceeding three or four triplets 

 may be had by the formula 



© + ©' + ©" + &c X + X' + &c 



n. n, — 1 



Where n is the number of sun observations, the time corres- 

 ponding being the middle of the middle shade observation. 



"A complete actinometer observation cannot consist of less 

 than three sun and two shade observations intermediate; 

 but the more there are taken the better, and 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 diurnal 

 increase and diminution of the solar radiation, which will 

 thus readily become apparent, provided the perfect clearness 

 of the sky continue — an indispensible condition in these ob- 

 servations ; the slightest cloud or haze over the sun being at 

 once marked by a diminution of resulting radiation. 



" To detect such haze or cirrus, a brown glass applied be- 

 fore 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 nothing of 

 the sort to be perceived by the unguarded eye. 



" It is, as observed, essential that the instrument be exposed 

 a few minutes to the sun, to raise its temperature in some 

 slight degree ; if this be not done, owing to some cause, not 

 very obvious, the first triplet of observations (sun, shade, 

 sun,) will give a radiation perceptibly in defect of the truth, 

 as will become distinctly apparent on continuing the series. 

 But it may be as well for a beginner to commence at once 

 reading as soon as the instrument is exposed, and reject the 

 first two triplets, by which he will see whether he has all his 

 apparatus conveniently arranged, and get settled at his post. 



