ACTINOMETER. lix 



0-924 inch ; thermometer tube, 6^ inches long ; the mercury, filling 2*9 inches of the 

 tube, weighed 11*5 grains, and 100 divisions of the scale are equal to 5*51 inches. 

 The screw, which is of silver, is 2\ inches long, and has 25 threads to an inch, the 

 diameter at the outer edge of the screw is 0-57 inch, and at the bottom of the screw 

 is 0*53 inch in diameter; it was not possible to determine the amount of heat 

 stopped by the outer cylinder, as both cylinders were screwed to the same end-piece. 

 95. In the summer of 1844, a new actinometer (with the old screw) was ob- 

 tained from the same maker. The cylinder and thermometer tube were inclosed in 

 a mahogany box, open at one side ; the compartment containing the cylinder filled 

 with the blue liquid is lined with black velvet, and is covered by a slip of plate- 

 glass. The dimensions were as follow : — Glass cylinder, 5^ inches long ; mean 

 external diameter, about 1*05 inch; the mercury, filling four inches of the thermo- 

 meter tube, weighed 16'7 grains; the length of 100 divisions of the scale are equal 

 to 5*51 inches. The cylinder of this instrument was again 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 dimensions 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 actinometer 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. The following are the results of series of observations for the 

 amount of heat stopped by the plate-glass used in the instrument after June 1844, 

 and marked A : — 



Sc. Div. 



1846. June l d 10 h 16 m A.M. Mean time. Glass plate A on ; mean effect of sun in 60 s = 9-47 



10 37 off =1204 



10 56 on = 9-70 



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



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



Sc. Div. 



1846. June 3 d 9 h 51 m A.M. Mean time. Glass plate A on ; mean effect of sun in 60 s = 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 60 s , glass plate A off =1285 



Mean on = 9-81 



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



