ACTINOMETER. Ivii 



to 5*51 inches. The cylinder of this instrument was destroyed in the winter of 

 1846-7, by the freezing of the liquid. The previous dimensions of the cylinder 

 belonging to the actinometer, from June 1844 till February 1847, are considered 

 to be very near the truth ; they are, however, only given from the dimension^ of 

 the cylinder in the actinometer at present, which is of the same size. The ac- 

 tinometer was placed in a small revolving frame during observations after June 

 1844, by means of which the face of the actinonleter was always presented to the 

 perpendicular incidence of the sun's rays ; at the end of the same table upon which 

 the revolving frame was placed, a double wooden screen was hung by cords passing 

 over pulleys ; the instrument could be shaded or exposed to the sun by the observer 

 instantaneously. 



95. In making an observation, the cylinder was exposed to the sun's rays at 

 a perpendicular incidence for 60 seconds, the scale readings of the fluid in the tube 

 being observed at the beginning and end of the minute. A screen was then inter- 

 posed for one minute, or for one minute and a half; if for one minute only, the last 

 observation in the sun was also noted as the first in the shade ; if for one minute and 

 a half, the first reading in the shade was not made till the instrument was shaded 

 half a minute. At the end of 60 seconds the scale reading was again observed, and 

 the screen was removed, that reading being also noted as the first in the sun. When 

 the liquid mounted near the top of the thermometer tube, the screw was withdrawn 

 nearly half a revolution, when the liquid fell to near the bottom of the tube. The 

 times were noted from a box-chronometer by Dent, No. 1665. 



96. The following are the results of series of observations for the amount of 

 heat stopped by the plate-glass, marked A, used in the instrument after June 

 1844 :— 



Sc. Div. 



1846. June 1'^ 10'' 16^ a.m. Mean time. Glass plate A on ; mean effect of sun in 60^= 9-47 



10 37 off =1204 



10 56 on = 9-70 



Mean effect of sun in 60s glass plate A on = 9"58 



Proportion of whole heat stopped by the glass plate A, =0'204. 



Sc. Div. 



1846. June 3"* 9^ 51«i a.m. Mean time. Glass plate A on ; mean effect of sun in 60^= 929 



10 12 off =12-83 



10 33 on = 9-82 



10 51 off =12-88 



11 10 on =10-33 



Mean effect of sun in 60s glass plate A off =12-85 



Mean on =9-81 



Proportion of whole heat stopped by glass plate A, = 0-237. 

 MAG. AND MET. OBS. 1845 AND 1846. p 



