Apkil 2, 1894.] 



KNOWLEDGE 



83 



the disc, and entered in column C. The throws of the 

 instrument are then got by subtracting the figures in 

 columns u, N, and C from the zero. The deflections of 

 the instrument have been experimentally proved to be 

 strictli/ jD-opartioiuil to the amount of radiant beat falling 

 on the thermo-couple. The following is a typical observa- 

 tion taken August 7th, 1893, of a large sunspot then visible. 

 The umbro of this spot measured 0'8 in. across on the 

 screen, so that the aperture of the radio-micrometer was 

 only covering about ^^th of that of the umbra. 



The ratio- 



umbra of spot 



1-31 



=0-292. 



neighbouring photosphere=j ,„ 



Five concordant readings gave a mean deflection of 

 4-57 for the centre of the sun, which gives for the ratio 

 umbra _Q.^g^ 

 centre 



This spot was at a distance from the centre of the disc 

 of about 0'4. 



As the radiation from the photosphere falls off from 

 the centre to the edge of the disc, it seemed an interesting 

 point to determine if any change in the ratio of u/C would 

 take place as a spot was carried across the disc by the 

 sun's rotation. If the spot is, as has been generally thought, 

 a depression, the absorption of heat ought to increase as it 

 is carried towards the limb, on account of the increased 

 depth in the solar atmosphere through which the radiation 

 would have to pass. On the other hand, if the spot was 

 floating above the absorbing atmosphere the radiation 

 from it would remain constant in any position on the solar 

 disc. 



The following is the value of the heat radiation from the 

 photosphere taken along a radius of the sun, where = 

 centre and 100 is the distance of the centre from the limb. 

 The radiation E equals 100 at the centre.'' 



D. R. D. R. 



1000 70 878 



10 99-8 75 85-3 



•ZO 99-5 80 82 5 



25.. 99-3 90 720 



30 98-9 95 ... 61-8 



40 97-2 98 51-3 



,50 953 100 429 



60 922 



It will be seen by the following observations of spots, 

 taken from August .5th to November 9th, that there is 

 distinct evidence that the radiation from the spot does not 

 fall off as rapidly when near the limb as the radiation from 

 the neighbouring photosphere; in fact, the ratio n/C 

 remains nearly constant, whereas the ratio u/'N gets nearer 

 unity as the spot approaches the limb. The spot 

 observed on October 22nd is a good example, as the 

 same spot was observed again on the 26th, 29th, and on 



* " The Absorption of Heat in the Solar .4tmosphere," by W. E, 

 Wilson and A. A. Rambaut, Proceedings of the Royal Irish Aeadetny. 

 3rd series, Vol. II., No. 2. 



the 30th, when it had reached within a distance, D, of 95 

 fi-om the centre. It will be seen that on these four dates 

 the ratio «/C was respectively 0-338, 0-360, 0-313, 0-356, 

 whereas the ratio it/N was 0-349, 0-410, 0-706, 0-783. 



Langley, I in 1871 and 1875, measured the radiation from 

 the sun and the radiation from spots. He used a thermo- 

 pile and galvanometer, and obtained as the mean of his 

 results a ratio of 0-54+0-05. 



His method was first to take a reading in the neighbour- 

 hood of the spot, but between it and the centre of the disc. 

 He then took a reading in the umbra, and, finally, a third 

 reading in the neighbourhood between the spot and the 

 edge of the sun. 



The mean of the two photospheric readings he used as a 

 divisor for the umbral reading. He then says, " The 

 decrement of heat as we approach the limb is, though not 

 exactly, yet so very nearly, in the same ratio for photo- 

 sphere and spots, that no correction is needed on this 

 account for the present observations." 



If Langley failed, through want of instrumental means, 

 to notice the difference between the absorption in a spot 

 and the photosphere near the limb, his method would make 

 his umbral readings too high. The mean of twenty obser- 

 vations here equals 0-356, against Langley's 0-54. This 

 is a serious difference, and, I think, can only be accounted 

 for either by the use of superior instrumental means, or by 

 a possible variation in the radiation of spots in different 

 years of the sunspot cycle. 



It is difficult to see how too loir a value for umbral radia- 

 tion could be got, whereas too high a one might be found 

 by want of definition and trembling in the image, so that 

 some of the penumbral radiation would reach the thermo- 

 couple. 



THE 



NORTH POLE OF THE MOON. 



By A. C. Ranyakd. 



WHATEVER may be the temperature of the 

 equatorial regions of the moon, it will not be 

 disputed that the moon's polar regions must 

 be exceedingly cold, for no clouds float above 

 the moon's surface, and many phenomena go 

 to prove that if the moon has an atmosphere it must be a 

 very thin and scanty one compared with our own, so that 

 such an atmosphere would aff'ord but a poor covering to 



t Monthly Notices, Vol. XXXVII., No. 1. 



