40 



ASTRONOMICAL PHENOMENA AND PROGEESS. 



hours from noon, (5) two hours from noon, (6) 

 one hour from noon, (7) noon. 



"Each of the first six of these groups con- 

 tains two separate sets of observations : (1) 

 those made before noon, (2) those made after 

 noon. It has already been pointed out, from 

 experiments made at Kew, that the mean 

 chemical intensity of total daylight for hours 

 equidistant from, noon is constant. The result 

 of the present series of experiments proves 

 that this conclusion holds good generally, and 

 a table is given showing the close approxima- 

 tion of the numbers obtained at hours equi- 

 distant from noon. 



" Curves are given showing the daily march 

 of chemical intensity at Lisbon in August, 

 compared with that at Kew for the preceding 

 August, and at Para for the preceding April. 

 The value of the mean chemical intensity at 

 Kew is represented by the number 94.5, that at 

 Lisbon by 110, and that at Par& by 313.3, light 

 of the intensity 1.0 acting for 24 hours being 

 taken as 1,000. 



" The following table gives the results of the 

 observations arranged according to the sun's 

 altitude : 



Curves are given showing the relation between 

 the direct sunlight (column 3) and diffuse day- 

 light (column 4) in terms of the altitude. The 

 curve of direct sunlight cuts the base line at 

 10, showing that the conclusion formerly ar- 

 rived 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 ex- 

 perimentally-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 experi- 

 ments than the present) for Kew, Heidelberg, 

 and Pard ; so that, although the chemical in- 

 tensity for the same altitude at different places 

 and at different times of the year varies ac- 

 cording to the varying transparency of the 

 atmosphere, yet the relation at the same place 

 between altitude and intensity is always rep- 

 resented by a straight line. This variation in 

 the direction of the straight line is due to the 

 opalescence of the atmosphere ; and the au- 

 thors show that, for equal altitudes, the higher 

 intensity is always found where the mean tem- 

 perature of the air is greater, as in summer, 

 when observations at the same place at differ- 



ent 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-men- 

 tioned method at Lisbon, and that calculated 

 for Naples by a totally different method." 



Spectrum qf a Sun-spot, April 9, 1870, 

 Prof. 0. A. Young, of Dartmouth College, in- 

 vestigated the spectrum of a large group of 

 spots a little north and east of the sun's centre. 

 He found the lithium, calcium, and titanium 

 lines strongly marked, and the sodium lines 

 clearly perceptible. The titanium lines were 

 very well defined, a circumstance at which 

 Prof. Young was surprised, as they are incon- 

 spicuous in the normal spectrum. The same 

 remark applies to the calcium lines in the spot- 

 spectrum. Many other lines, mostly faint, 

 were affected to nearly the same degree, but 

 the observer had not time to identify them. 

 There was, at the same time, an exceedingly 

 brilliant protuberance on the southwest limb 

 of the sun (position angle 230), near, but not 

 over, a large spot which was just passing off. 

 At the base "of this prominence, which was 

 shaped like a double ostrich-plume, the C line 

 was intensely brilliant, so that the slit could 

 be opened to its whole width in studying the 

 form above described, but it was not, so far as 

 he could see, in the least distorted. On the 

 other hand, the F line, also very brilliant, was 

 shattered all to pieces, so that at its base it 

 was three or four times as wide as ordinary, 

 and several portions of it were entirely de- 

 tached from the rest. 



Since the C line was not similarly affected, 

 it is hardly possible to attribute this breaking 

 up of F to cyclonic motions in the gas from 

 which the light emanates, and it becomes very 

 difficult to imagine a cause which can thus dis- 

 turb a single line of the spectrum by itself. 

 Prof. Young suggests that possibly this ap- 

 pearance may be the result of local absorp- 

 tions acting upon a line greatly widened by 

 increase of pressure or temperature. 



The Kew Heliograph. Mr. J. P. Sassiot, 

 chairman of the committee of the Kew Ob- 

 servatory, has made a report of the work done 

 at that institution during the past year. The 

 heliograph in charge of Mr. Warren De La 

 Rue continued to be operated in a satisfactory 

 manner. In 237 days 351 pictures of the sun 

 were taken. A paper embodying the positions 

 and areas of the sun-groups observed at Kew 

 during the years 18C4, 1865, and 1866, as well as 

 fortnightly values of the spotted solar area from 

 1832 to 1868, has been communicated to the 

 Royal Society by Messrs. Warren De La Rue, 



