196 S. P. Langley—The Actinic Balance. 
measures was an old and comparatively rough one, adjusted 
as nearly to insensitiveness as possible by its directing magnet, 
so that the image should not be thrown off the scale. The 
actual readings are given of a series of seven measurements on 
the transmissibility of solar rays by water and by a solution of 
common alum containing ten per cent by weight, as taken by 
Mr. F. W. Very, on Sept. 22d, 1880, 11 A.M. tol P.M | 
The liquids being enclosed in glass cells (sides 25™ thick, 
distance between sides =19-0™™) were interposed or withdrawn 
by sliding the stand, on which the cells were held perpendicular 
to a sunbeam, so that the center of the circular cell should all 
opposite the aperture of the actinic balance case, which was 
inclined so as to point to the sun. Hach reading on the sun 1s 
the mean of two taken directly before and after the interposl- 
tion of the liquid. 
1) Per cent 
Radiation of fall Fe er St || padintion ot fall | FROME" | orenanttted. 
Sun. Sun. e 
205 : 217 53°9 
215 i 210 123 58°6 987 { 282 152 
214 : 291 53°3 
219 217 127 58°5 272 287 153 
226 : 275 51°4 
939 233 132 56°T 240 253 130 
253 149 589 || Sas t 283 150 53°0 
225 135 600 |] Seo 279 148 63°1 
286 : 283 52:9 
289 288 174 60°4 265 274 145 
299 : 270 53°3 
284 292 178 61°0 248 259 138 | ae 
lar radiation transmitted by water and glass = 59-2 per cent + “36. 
Probable error of one observation = + 0°96 per cent. 
Solar radiati i 
on transmitted by alum and glass = 53°0 per cent + ‘19. 
Probable error of one observation = + 0°51 per cent. 
These readings are given merely as fair samples of the average 
(not of the best) work where sensitiveness is not demanded. 
As an instance of a by far more delicate class of measure 
lens and a smaller condensing lens near the focus. A ered 
variety of precautions were used which are not here detatle¢, 
