Dec. 9, 1875J 



NATURE 



115 



it will thus be seen that, owing to the absorptive power of the 

 terrestrial atmosphere, the radiant intensities transmitted from 

 the opposite poles of the luminary differ considerably. The 

 magnitude of this difference, adequate to explain the discrepancy 

 under consideration, need not excite surprise if we consider that 

 thirty-two minutes of zenith distance involves an additional depth 

 of more than half a mile of atmosphere to be penetrated by the 

 ra3rs projected towards the actinometer from the j£7/<M pole of 

 the sun. The foregoing facts show the necessity of taking the 

 difference of zenith distance between the opposite 

 poles into account in making exact observations 

 of the sun's polar temperature, especially at the 

 iower altitudes where the secant of the zenich 

 listance increases rapidly. 



Regarding the calorific energy of the radia- 

 tion emanating fiom the border of the sun, I 

 deem it proper to present the following brief 

 statement. Several obser\'ations during the early 

 part of the investigation pointed to the fact that 

 increased energy is transmitted to the aciino- 

 meters by radiation from the sun's border. 

 Again, considerable irregularity was observed 

 in the progressive diminution of the force of 

 radiation towaids the circumference of the solar 

 disc. It was shown in the preceding article 

 (vol. xii, p. 520) that the radiation from the border 

 zone, i' 42 "* wide, occupying one-fifth of the 

 area of the solar disc, transmits o"638 of tlie 

 intensity transmitted from an equal area at the 

 centre of the disc. Of course it will be supposed that the rate 

 of the diminution of intensity within the zone thus ascertained 

 is much greater near the border of the photosphere than at the 

 middle of the zone. Such, however, is by no means the case, 

 notwithstanding the assumption of phycisists that the heat trans- 

 mitted by radiation from the border is very feeble. In order to 

 test the truth of the indications referred to showing considerable 

 radiant energy at the border of the photosphere, a very careful 

 investigation was made, Sept. 9, 1S75, by means of screens 

 excluding the rays from the solar disc, as shown in Figs. 12 and 

 13. The diameter of the screen represented in Fig. 12 being 

 154 06 millimetres, covered nine-tenths of the area of the disc ; 

 while the screen shown in Fig. 13, being 145 "25 millimetres, 

 covered four-fifths of the disc. It will be well to mention that 

 the dimensions of the screens referred to correspond with the 

 angle subtended by the sun when the earth is in aphelion. Ac- 



was adjusted previous to the observation, viz. shortened, in 

 order to compensate for the increase of the angle subtended by 

 the sun. Agreeable to the sUted dimensions of the screens, it 

 mil be found that the zone represented in Fig. 13 is i' 42', 

 while the zone in Fig. 12 is 49" '6. The mean width of the 

 latter is consequently situated only 24' '3 from the border of the 

 photosphere. 



The following table shows the intensities transmitted to the 

 actinometers from the zones represented in Fi^s. 12 and 13 : — 



Time. 



4' 



5' 



6' 



Zone. F;g. 1- 

 Ceiit. 



2° 01 1 



2° 248 



2° -425 

 2° 485 



Zon-, Fi?. 12. 

 Cens. 



Kate of difference. 



jjj — OCO2 

 2 01 1 

 IJ47I ^ 0654 



2 248 



^ t = o 652 



2-425 ^ 



1^66 



2 485 = 0670 



Mean = o'66o 



The rate of difference inserted in the last column, it will be 



noticed, is not quite so consistent as in the table contained in the 



cordingly the distance between the actinometers and the screens i previoos article recording the observations made Aug. 25. The 



discrepancy is, however, not material, the difference between the 

 lowest and the mean rate being oco8. It wLl be s«en on 

 inspecting the registered intensities, that the border zone repre- 

 sented in Fig. 12, whose area is only one-half of the area of the 

 zone in Fig. 13, transmits 0660 ol the intensity of the latter. 

 This at first sight indicates an extremely disproportionate 

 transmission of beat from the narrow border zone ; but it should 

 be considered that the inflected radiation imparts relatively 

 more heat to the actinometer exposed to the radiation from the 

 narrow zone than from the wide zone. It will be readily un- 

 derstood that since the inflection of the calorific rays is I4""7 

 (see preceding article, page 519), the first -mentioned actinometer 

 receives radiant heat from 14' 7 -J- 49' '6 = 64" 3 ; while the 

 aclinomettr exposed to the radiation from the wide zone re- 

 ceives heat from I ' 42" + 14'' 7 = Ii6""7. Consequently, the 



64" '3 _ 

 ii6''7~ 



o 551 of that transmitted from the wide zone, hence somewhat 

 more than one-halt. Our investigation therefore proves that the 

 radiant heat transmitted from the narrow border zone represented 

 in Fig. 12 is 0"66o - 0'55l = 0109 more intense than that 

 transmitted firom the zone represented in Fig, 13, although the 



radiant heat emanating from the narrow zone will be 



mean distance of the latter is twice as far from the border of the 

 photosphere as the mean distance of the former. The singular 

 fact thus revealed can only be accounted for by supposing that 

 internal radiation is not incompatible with the constitution of the 

 photosphere, and by adopting Lockyer's views expressed in the 

 Set ate House at Cambridge, 1871, that "the photosphere 

 must be a something suspended in the solar atmosphere." Let 

 a b. Fig. 14, represent a section of the " suspended " photo- 

 sphere, and dc, gf, rays projected towards the earth. Agree- 

 able to the conditions mentioned, and in view of the fact that 

 the force of radiation from incandescent gases presenting equal 

 areas, varies nearly as their depth, we are warranted in con- 

 cluding that since the depth dd' is greater than gg", the radiant 

 heat transmitted from the photosphere by the ray dc will be 

 greater than that transmitted by the ray g/. It should be 

 observed that the energy transmitted towards the earth by dc 

 suffers a greater diminution than the energy transmitted hy £ fin 

 consequence of the greater depth of the solar atmosphere 

 penetrated. Hence the augmented energy established by our 

 investigation, does not show the full amoimt of the increase of 

 radiant heat transmitted from the border of the sxm. 



1 J. Ericsson 



