58 Royal Society .— 



The following Table gives the results of the observations arranged 

 according to the sun's altitude. 



Chemical Intensity. 

 No. of observations. Mean altitude. Sun. Sky. Total. 



15 9 51 0-000 0-038 0-038 



18 19 41 0-023 0-063 0-085 



22 31 14 0-052 0*100 0*152 



22 42 13 0-100 0-115 0-215 



19 53 09 0-136 0-126 0-262 



24 61 08 0-195 0-132 0-327 



11 64 14 0-221 0-138 0359 



Curves are given showing the relation between the direct sunlight 

 (column 3) and diffuse daylight (column 4) in terms of the altitude. 

 The curve of direct sunlight cuts the base line at 10°, showing that the 

 conclusion formerly arrived at by one of the authors is correct, and 

 that at altitudes below 10° the direct sunlight is robbed of almost all 

 its chemically active rays. The relation -between the total chemical 

 intensity and the solar altitude is shown to be represented graphically 

 by a straight line for altitudes above 10°, the position of the experi- 

 mentally determined points lying closely on to the straight line. 



A similar relation has already* been shown to exist (by a far less 

 complete series of experiments than the present} for Kew, Heidel- 

 berg, and Para ; so that although the chemical intensity for the 

 same altitude at different places and at different times of the year 

 varies according to the varying transparency of the atmosphere, yet 

 the relation at the same place between altitude and intensity is always 

 represented by a straight line. This variation in the direction of the 

 straight line is due to the opalescence of the atmosphere ; and the 

 authors show that, for equal altitudes, the higher intensity is always 

 found where the mean temperature of the air is greater, as in summer, 

 when observations at the same place at different seasons are compared, 

 or as the equator is approached, when the actions at different places are 

 examined. The differences in the observed actions for equal altitudes, 

 which may amount to more than 100 per cent, at different places, and 

 to nearly as much at the same place at different times of the year, 

 serve as exact measurements of the transparency of the atmosphere. 



The authors conclude by calling attention to the close agreement 

 between the curve of daily intensity obtained by the above-mentioned 

 method at Lisbon, and that calculated for Naples by a totally different 

 method. 



April 7. — Dr. William Allen Miller, Treasurer and Vice- 

 President, in the Chair. 



The following communications were read : — 

 "On Supra-annual Cycles of Temperature in the Earth's Surface- 

 crust." By Professor C. Piazzi Smyth, F.R.3. 



The author presents and discusses the completely reduced obser- 



* Phil. Trans. 1867, p. 555. • . 



