ACTINOMETER. lvii 
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 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. 
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the liquid mounted near the top of the thermometer tube, the screw was withdrawn 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 :— 
Se. Div. 
1846. June 1410"16™ a.m. Mean time. Glass plate A on; mean effect of sun in 60°= 9:47 
1) 27 “e gogtbsetsnan oon sen snab0Gsce ORES cfrepartoehcilee sislerelanieks =12:04 
I) Sit” seonkgeenen Dacre nda caeaes DE Gn isarec AA sap Ren SDD aC = 9-70 
Mean effect of sun in 60°, glass plate A on = 9:58 
Proportion of whole heat stopped by the glass plate A, =0°204. 
Sc. Div. 
1846. June 34 9°51™ a.m. Meantime. Glass plate Aon; mean effect of sun in 60°= 9:29 
WU, occ oO ONO SB ODO ADO OSU CE ms Oorne Olt de dtc Subbaoots np oot =12'83 
10) 33) CSA ASA an aS ore eats ODM. fre svc olateterareteietarete ates = Sey 
ON aaa yar to 2 ctals yatoyare tare ara abla tahatan lee Oe Re Ran Sc Eo abr ob =12:88 
IN UO) Coe eeor Saseptece ne ern tone ve Ol ee aaanedeeseccmagl coat = 10°33 
Mean effect of sun in 605, glass plate A off =12°85 
IWIGET \Coosec MERC RE Ieee ee eea one dee on = 981 
Proportion of whole heat stopped by glass plate A, = 0:237. 
MAG. AND MET. OBS. 1845 AnD 1846. p 
