﻿28 Mr. Carl Bar us on 



enter the present problem, and that B = p has already been 

 disposed of. Then the pressure p m , corresponding to the 

 minimum temperature t m in question, is found to be 



B 



and the somewhat more involved expression of t m is found from 

 equation (1). 



With these preliminaries, the remaining constants C and n 

 are then easily enough, though somewhat tediously, obtained 

 from the observations making up fig. 5, by trial. The results 

 are as follows : — 



A=9; C = 0-013; n = 0'35; B = 43. 



Steam-pressures, p, in cm. of mercury ; air-temperatures, t, 

 in degrees C. 



p= Ocm. t= 9-0° p= 43 cm. f=+oo° 



10 9-8 44 33-6 



20 11-0 47 21-4 



30 13-0 50 19-2 



40 20-3 *66-2 *17-4 



42 31-6 70 17*4 

 42-8 85-3 90 18*1 



43 +oo 



^ j < j 



Blue-opaque. * Minimum. Yellow-opaque. 



This curve, equation (1), has been inscribed in fig. 5, to 

 show the grouping of the observations around it. The mini- 

 mum is marked at a (t m = 17*4° C, p m = 66'2 cm.). Through- 

 out the extent of the figure, it unites two sufficiently flat curves 

 to fairly represent the observations ; for this part of the 

 margin, from its exceedingly steep ascent, cannot be traced 

 with precision. 



As a whole, therefore, equation (1) has reproduced the 

 complete phenomenon surprisingly well, both as regards the 

 blue-opaque (AB) and the yellow-opaque (BO) margin of 

 the opaque field. No doubt, better agreement could be had 

 on further trial, particularly by varying the point of inter- 

 section with the abscissa, £=A. I shall not, however, do this, 

 since in the present paper the chief datum is the height of the 

 common asymptote (p = B) above the abscissa. It is this 

 parameter which expresses the dust-Contents of the air, and 

 which fortunately may be obtained without computation by 

 the direct observations presently to be more fully specified. 



11. Artificially Dusty Atmospheres. — To interpret the above 

 data it is necessary to increase the dust-contents of the normal 



