130 



KNOWLEDGE 



[June 1, 1895. 



Dr. Donaldson Smith, who is on a scientific expedition 

 to Lake Rudolph from the Somali coast, has certainly 

 made a discovery that may rival in interest that 

 of the wonderful sculptured stones, rock-buildings and 

 pictographs on Easter Island. On his way south from the 

 Shibeyli river he explored a river bed that ran under a 

 mountain, and at the head of which he had learnt " the great 

 god of the Gallas tribe had carved a palace to himself." 

 He determined to solve the mystery, and his pluck was 

 certainly amply rewarded. " We discovered," he writes 

 to friends in America, "the most beautiful subterranean 

 passages it would be possible to imagine. A large tributary 

 of the river -Juba had carved a way for itself under a 

 mountain a mile in length. On either side of the stream 

 were great vaulted chambers from twenty to forty feet 

 high, and supported on massive columns." Had the 

 learned reporter stopped here, speculation would have been 

 confined to geological lines, and the explanation not a 

 difficult one. But Dr. Smith proceeds to say that " the 

 columns were most beautifully carved, and many of them 

 joined, forming long arched passages. The mountain was 

 hollowed out a great distance on either side of the stream." 

 Fuller details of this very extraordinary discovery will be 

 awaited with interest. 



At a recent meeting of the Eoyal Meteorological Society, 

 Mr. W. W. Shaw, F.E.S., delivered a lecture on " The 

 Motion of Clouds," considered with reference to their 

 mode ot formation, which was illustrated by experiments. 

 The question proposed for consideration was how far the 

 apparent motion of a cloud is a satisfactory indication of 

 the motion of the air in which the cloud is formed. The 

 mountain cloud-cap was cited as an instance of a stationary 

 cloud formed in air, moving sometimes with great rapidity. 

 The two causes of formation of cloud were nest considered — 

 (1) the mixing of masses of air at different temperatures, 

 and (2) the djTiamical cooling of air by the reduction of 

 its pressure without gain of heat from the outside. A 

 sketch of the supposed motion of the air near the centre of 

 a cyclone, showed the probability of the clouds formed by 

 the mixing of air being carried along with the air after 

 they were formed, while when cloud is being formed by 

 expansion, circumstances connected with the formation of 

 drops of water on the nuclei to be found in the air, and 

 the maintenance of the particles in a state of suspension 

 make it probable that the apparent motion of such a cloud 

 is a bad indicator of the motion of the air. The meteoro- 

 logical effects of the thermal disturbance which must be 

 introduced by the condensation of water vapour was 

 referred to, and to this cause was attributed the violent 

 atmospheric disturbances which accompany tropical rains. 



At the annual Royal Society convfisa-Jone in May, 

 phenomena associated with cloud formation were experi- 

 mentally illustrated by Mr. Shaw. Clouds formed by 

 mixture of two currents of air were shown in a large glass 

 globe. The currents were due to convection. The motion 

 of the clouds gave an indication of the motion of the air. 

 Under suitable conditions the motion took a gyratory or 

 " cyclonic " form. A second globe was arranged to show 

 the formation of a cloud by the dynamical cooling of air, 

 consequent upon a sudden expansion equivalent to an 

 elevation of about ten thousand feet. The water globules 

 could be seen to fall slowly. A light was arranged at the 

 back of the globe to show (under favourable circumstances) 

 coloured coronas surrounding a central bright spot. 



atmosphere of Mars is greater than that in the earth's 

 atmosphere, it is useless to look for the presence of water- 

 vapour by means of the spectrum of Mars, unless the 

 instruments employed are much superior to any hitherto 

 used for that purpose. The chances of detecting the 

 presence of oxygen do not seem so hopeless, and it is 

 suggested that indications of the presence of the green 

 colouring matter of plants might be got. 



Mr. .Jewell, from his investigations made at Baltimore, 

 concludes that unless the amount of water in the 



THE SUN'S STELLAR MAGNITUDE. 



By J. E. Ctore, F.E.A.S. 



THE stellar magnitude of the sun is the number 

 which represents its brilliancy on the scale in 

 which the magnitudes of the stars are represented. 

 In this scale the "light ratio," as it is termed, is 

 now generally taken at 2-512, of which the 

 logarithm is 0-4. This light ratio denotes that a star of 

 the first magnitude is 2-512 times brighter than a star of 

 the second magnitude, a star of the second magnitude 

 2-512 times brighter than one of the third, and so on. 



In ancient times all the brighter stars were classed 

 together as of the first magnitude, but as many of the so- 

 called first magnitude stars, as Sirius, Arcturus, Vega, 

 Capella, etc., are considerably brighter than other first 

 magnitude stars, like Altair, Aldebaran, Spica, etc., this 

 classification is not sufficiently accurate for the require- 

 ments of modern science. These very bright stars are, 

 therefore, now considered as brighter than the first 

 magnitude, and their brightness is sometimes represented 

 by a decimal fraction, the scale thus beginning from or 

 zero. Sirius, the brightest star in the heavens, has 

 been found by photometric measures to be about two 

 magnitudes brighter than an average star of the first 

 magnitude, like Altair or Spica, and its stellar magnitude 

 may, therefore, be represented by — 1-0, or one magnitude 

 brighter than the " zero magnitude." Now, what figure 

 would represent the brightness of the sun on this scale '? 

 The sun's brightness is so vastly greater than even a star 

 like Sirius, that it might be supposed that a very large 

 number would be required to represent its brightness in 

 the stellar scale of magnitudes. This, however, is not the 

 case. As wiU be seen, the relative brightness of the stars 

 in the assumed scale forms a geometrical series, and 

 increases very rapidly. Thus, a star of the average first 

 magnitude is one hundred times brighter than one of the 

 sixth, and ten thousand times brighter than a star of the 

 eleventh magnitude, and so on. 



Various attempts have been made to determine the sun's 

 stellar magnitude, but owing to its excessive brilliancy its 

 accurate determination is a matter of no small difficulty. 

 Comparing its light with that of the moon, WoUaston, in 

 1829, found it 801,072 times brighter ; Bond, in 18U1, 

 found 470,000, and (by another method) 340,000 ; and 

 ZiiUner found 618,000. These results are rather discordant, 

 but Zollner's figure has been usually accejited as the most 

 reliable. Huygens found that the sun is 22-2 magnitudes 

 brighter than Sirius ; Wollaston, 25-75 ; Steinheil, 23-96 ; 

 Bond, 24-44 ; and Clark, 23-89. The arithmetical mean 

 of these determinations is 24-05. If, however, we omit the 

 value found by Huygens, which is evidently too low, we 

 have a mean of 24-51. Taking the magnitude of Sirius at 

 -1-0, we have the sun's stellar magnitude —25-51. Prof. 

 Pickering adopts 25-5, and this is the value which I have 

 assumed in my previous papers in Knowledge. 



As there seems to be an uncertainty as to the accuracy 

 of this value, the followmg method of computing it has 

 been suggested to me by my friend Mr. Monck. Taking 



