520 



NATURE 



\Oct. 14, 1875 



exact moment for reading off being indicated by a chronograph* 

 The relative intensities transmitted by radiation from the centr^ 

 and from the border of the solar disc, first claim our attention- 

 Fig. 6 represents the solar disc covered by a circular screen 

 145-25 millimetres in diameter, excluding the rays excepting 

 from a narrow zone, the mean width of vi'hich is situated 49" 

 from the border ot the photosphere. Fig. 7 shows a screen 

 excluding the solar rays excepting from the central portion, the 

 area of which is precisely equal to the area of the narrow zone in 

 Fig. 6. The following table shows the intensities transmitted to 

 the actinometers during an observation, August 25, 1875, the 

 radiation from the solar disc being then excluded in the manner 

 shown in Figs. 6 and 7 : — 



It should be particularly observed that this table records the 

 result of four distinct observations ; nor should it be overlooked 

 that although the intensities vary greatly for each observation in 

 consequence of the continued exposure to the sun, yet the rates 

 showing the difference of the intensity of the rays transmitted 

 from the border, inserted in the last column, is practically the 

 same for each observation, the discrepancy between the highest 

 and the lowest rate being only 0-004.* Persons practically 

 acquainted with the difficulty of ascertaining the intensity of 

 solar radiation will be surprised at the exactness and consistency 

 of the indications of our actinometers. This desirable exactness 

 has been attained by surrounding the actinometers with water- 

 jackets, which communicate with each other by connecting pipe?, 

 through which a steady stream of water is circulated. By this 

 expedient the chambers containing the bulbs of the several ther- 

 mometers are maintained with critical nicety at equal tempera- 

 ture, an inexorable condition when the object is to determine 

 differential temperature with great exactness. Apart from this, 

 the chambers which contain the bulbs of the thermometers are 

 air-tight, the radiant heat being admitted through a small aper- 

 ture at the top of the chamber, covered by a thin crystal. 



Referring to the preceding table, it will be seen that the in- 

 tensity transmitted by radiation from the sun's border, repre- 

 sented in Fig. 6, is 0-667 of the intensity transmitted from the 

 central region represented in Fig. 7, the area of each being pre- 

 cisely alike. From the stated intensity must be deducted the 

 heat imparted to the actinometer by the inflection of the calorific 

 rays. The circumference of the perforation of the screen shown 

 in Fig. 7 being exactly one-half of the circumference of the 

 screen in Fig. 6, while the central region radiates more power- 

 fully than the border, fully one-half of the inflected radiation 

 from the border will be balanced by the inflected radiation 

 emanating from the central region. Agreeable to the previous 

 demonstration relating to Figs. 2 and 3, it will be seen that the 

 unbalanced inflection amounts to 0*029 5 hence the radiation trans- 

 mitted from the border zone will be 0667 — 0-029 = o'638 of 

 the intensity of radiation transmitted from the central region. 

 We have thus shown by a reliable method that the intensity of 

 the rays directed towards the earth from the border zone suffers 

 a diminution of I'coo — 0638 — 0-362 of the intensity of the 

 radiation emanating from the central region. But the mean 

 depth of the solar atmosphere of the border zone, in the direc- 

 tion of the earth, is 2-551 greater than the vertical depth, while 

 the mean depth over the central region referred to is only 0036 

 greater than the vertical depth of the solar atmosphere. Conse- 

 quently, if we accept the assumption that the retardation is as 

 the depth, the absorption by the solar atmosphere cannot exceed 



5-^ = 0*144 of the radiant heat emanating from the 



2-551-0-036 ^ ^ 



* All my instruments for measuring radiant heat have been graduated to 

 the Fahrenheit scale, which practically is more exact than the Centigrade, 

 owing to its finer divisions. For the benefit of the Continent.il readers of 

 Nature, and in order to satisfy English and American advocates of the 

 course Centigrade, the observed temperatures have been reduced to that 

 cale before being entered in our taWes. 



photosphere.* It will be found, on referring to the revised 

 edition of "Le Soleil," vol. i. p. 212, that P^re Secchi 

 makes the following statements regarding the absorptive power 

 of the solar atmosphere, (i) "At the centre of the disc, 

 that is to say perpendicularly to the surface of t1:e pho- 

 tosphere, the absorption arrests about f or more exactly 

 tVt <-'f ttie total force." (2) "The total action of the ab- 

 sorbing envelope on the hemisphere visible from the sun is 

 so great that it allows only tircr of the total radiation to pass, 

 the remainder, namely, //ir. being absorbed." It is unnecessary 

 to criticise these figures presented by the Roman astronomer, 

 as a cursory inspection of our table and diagrams is sufficient 

 to show the fallacy of his computations. Apart from deter- 

 mining the absorptive power of the solar atmosphere, the most 

 important problem which may be solved by accurately measur- 

 ing the intensity of the radiation emanating from various parts 

 of the disc, is that relating to the sun's emissive power in dif- 

 ferent directions. In order to decide this question, I have 

 adopted the plan of measuring the energy of the radiant heat 

 transmitted from zones crossing the solar d'sc at right angles, 

 as shown in Figs. 10 and 11. Should it be found that our 

 actinometers are equally affected by the radiation from these 

 zones, each of which occupies an arc of 30 deg- containing one- 

 third of the area of the disc, the inference will be irresistible 

 that the sun emits heat of equal intensity in all directions. It 

 should be borne in mind that, agreeable to our method, the 

 radiations from these zones are observed simultaneously. The 

 arrangement exhibited in Figs. 10 and 11 hardly needs explana- 

 tion. Referring to Fig. 10, it will be seen that two segmental 

 screens are employed excluding the radiant heat, excepting 

 from tlie zor.e, which is parallel with the sun's equator. 

 Similar screens are employed (see Fig. Ii) for excluding the 

 rays excepting from the zone parallel with the sun's polar axis. 

 The curvatures of the segmental screens, it should be observed, 

 have been struck to a radius of ninety millimetres, in order to 

 cut off effectually the inflected radiation from the suh's border. 

 Obviously diffraction has not called for any correction of our 

 observations relating to this part of the investigation, since the 

 inflected radiation from the equatorial zone exactly balances the 

 inflected radiation from the polar zone. It only remains to be 

 stated that repeated observations show that the radiant energies 

 transmitted to the actinometers from the two zones are iden- 

 tical. The result of observations relating to the radiation 

 emanating from the polar regions, represented in Figs. 8 and 9, 

 together with other observations, will be discussed in future 

 communications. J. Ericsson 



SOME LECTURE NOTES ON METEORITES^ 

 III. 



A MONG the mineral constituents of meteorites the tmstable sul- 

 -^^ phides, it is hardly necessary to observe, could with difficulty 

 be conceived as continuing permanently undecomposed, or as being 

 even formed under the ordinary conditions of reck formation on 

 our globe ; and the same remark may be extended, though with 

 some limitation, to the metallic iron that is so characteristic and 

 ubiquitous a constituent of almost every, if, indeed, not (as main- 

 tained by Dr. Lawrence Smith) of every meteorite. On the other 

 hand, it is to be remembered that the rocks that we are acquainted 

 with on our globe are only those composing its outer crust; 

 rocks which represent the results of the corrosive action of the 

 atmospheric agencies, oxygen, carbonic acid, and water, and 

 their counterpart the ocean, on whatever material the con- 

 solidated surface of our planet offered for their action. The 

 endless cycle of mechanical and chemical disintegration, decom- 

 position, and reconstruction would be limited to a shallow shell, 

 and even the fresh matter forced out to the surface in volcanoes, 

 through the contraction of the cooling globe, would consist in 

 all likelihood only of the lower-lying layers of an already to a 

 certain degree metamorphosed material. Whether the inner 

 core of this planet is still in the meteoric condition — that is to 

 say, still may contain such minerals as native iron, associated 

 with nickel, not to say magnesium or calcium sulphides, is a 

 question not to be lost sight of in explaining the high specific 

 gravity of our globe as compared with that of the rocks that 

 form its crust. 



* In the first edition of " Le Soleil," p. 264, the author assumes that the 

 absorption of the calorific rays by the atmosphere " augments in proportion 

 to the secant of the zenith distance ;" in other words, as the depth of the 

 atmosphere penetrated by the rays. 



t Concluded from p. 507. 



