156 



KNOWLEDGE, 



[July, 1903. 



Let me add to the above, that Lord Kelvin's si)-called 

 theoretical proof of the sun's inability to raise a terrestrial 

 magnetic storm has never lirouglit a grain of conviction to 

 my self-reliant mind, and therefore I do not feel called 

 upon to look for electrified corpuscles outside the sun, whicli 

 are credited by some as taking part in the intervention of 

 the forces concerned. This is very much like robbing 

 Peter to pay Paul, apart from the fact that tlie observational 

 objections to which I have already referred are in no way 

 satisfied b;' such and similar propositions. Be it also 

 lemembered, that one authentical case disposes of Lord 

 Kelvin's '• impossilile " dictum. In my opinion, the whole 

 process is one of " induction," and the amount of work to 

 bo done by the sun to raise a terrestrial magnetic storm is 

 merely a question of the sim's electric potential where 

 disturbances occur, and of electrical conductivity of the 

 luminiferous ether lietween sun and earth, both iitterly 

 unknown quantities to Lord Kelvin. 



Albert Alfred Buss 



9, Grosvenor Square, Ashton-on-Mersey 



CIRCTJLAK RAINBOWS. 



TO THE EDITORS OF KNOWLEDGE. 



Sirs, — In reading over the correspondence Lu your 

 columns on " Kainbow " in the Feliruary, Septemlier, and 

 October munbers of Knowledge, 1901, 1 thought it might 

 lie worth while mentioning one thing I have noticed about 

 rainbows while at sea, that is, that under certain conditions 

 the rainbow is seen as a complete circle. The conditions 

 required are. the oliserver should be situated on a mast or 

 s<iniething of small diameter which does not prevent the 

 sun from shining on the rain falling in the vicinity ; he 

 must be at least about 50 feet above the sea level ; the 

 higher he is the more brilliant will the colour appear of 

 that part of the circle projected on the sea. But I have 

 found that at a height of about 150 feet, the colour of the 

 part with the sea or the deck of the ship for a background 

 is cjuite as brilliant as the coloiu- in the part projected on 

 the clouds, which forms the rainbow to an observer at the 

 sea level. 



Then as regards the sun's altitude ; if it is over 42° then 

 the w'hole circle is projected on the sea. I have seen this 

 circle where the sun had an altitude of over 75°, but then 

 the circle crossed the vessel's deck. Of course it could loe 

 seen with the sun in the zenith, but it would be almost 

 inqiossible for it to l>e raining and the sun to !« shining 

 in the same spot where the sun is at that altitude. 



Although I have spoken of this " rainbow " as a circle, 

 it appears as a circle only when the sun's altitude is over 

 42°. When the sun's altitude is less than 42^, and part is 

 projected on the sky and part on the sea, then it appears 

 to be an ellipse, a line drawn from the centre of the bow 

 proper to that part of the ellipse close to the observer 

 forming the major axis. "When there has been a secondary 

 bow visible from the deck, I have often gone aloft high 

 enough to see the primary as a complete elHpse, but have 

 never seen any more of the secondary bow than was visible 

 from the deck. 



In writing this I may only be rejieating what has often 

 been noticed before, but I have never seen it mentioned 

 anywhere. Charles C. Dixon. 



Liverpool. 



born at Newcastle on August 7th, 1841, and eventually 

 settled down in London as an engineer ; but in addition to 

 his business life, he devoted much time and energy to 

 astronomy, especially to the construction of large reflecting 

 telesco])es. Early in life he possessed a 5i-inch refractor, 

 and in 187(5 he was observing with an 18-iuch reflector; 

 but not satisfied with this, he t>onstructed a 3-ft. 

 equatorial reflecting telescope, with a silver-on-glass 

 reflecting surface. A slight delay was exjjerienced in 

 getting this instrument to work, as the first mirror burst 

 into fragments, but in 1879 the second mirror was success- 

 fully mounted. Recognizing the value of photography, 

 he had already applied it to obtain records of the Pleiades 

 and other regions, but with this increased optical power 

 he at once turned his attention to secure impressions of 

 nebula;, and it was with this instrument that he obtained 

 the splendid photograph of the Nebula in Orion, gained 

 an astronomical reputation, and won the gold medal of the 

 Royal Astronomical Society. Other observations with the 

 8-ft. enabled an error in the ephemeris of Mimas, the 

 satellite of Saturn, to be corrected. However, with 3-feet 

 aperture the ambition of Dr. Common was not yet reached, 

 for in 1880 he ordered a 61- inch disc of glass, and 

 eventually successfully completed his 5-ft. reflector, which 

 perhaps, with a newly-silvered mirror, is the most light- 

 grasping instrument ever constructed, and although it 

 never established the record that the smaller obtained, 

 and has again recently secured at the Lick Observatory, 

 yet it is doubtful if some of the unpublished photographs 

 of Jupiter and Saturn taken with it have been surpassed. 



Space will not permit mention of more than a few of 

 the mechanical difficulties he overcame in these mammoth 

 instruments — the successful silvering of large mirrors, the 

 floating of the polar axis to reduce friction, and the slipping 

 plate to correct clock irregularities. 



He was elected a Fellow of the Royal Astronomical 

 Society in 1876, and of the Royal Society in 1S85. and 

 was President of the former Society in 1895-6. He was 

 an honorary ll.d. of St. Andrews, and filled many offices 

 in connection with astronomy. 



Of late years his energies had teen largely devoted to 

 the application of the telescope to gun-sighting, and he 

 was so successful that the War Office and Admiralty had 

 adopted his gun-sighting telescope. He had many other 

 improvements in sighting apparatus in contemplation, and 

 his death is not only a loss to the astronomical wtirld, luit 

 to the nation. 



•ijttuar^. 



ANDREW AINSLIE COMMON, ll.d., f.r.s. 

 We much regret to announce the death of one who has 

 done so much for astronomical science as Dr. Common, 

 who died verv suddenlv at Ealing on June 2nd. He was 



Astronomical. — Prof. Barnard has recently jjublished 

 a very interesting account of observations of the south 

 polar cap of Mars which he made at the Lick Observatory 

 in 1892 and 1894. He considers the early supposition 

 that the caps are accumulations of snow to be as good as 

 any theory since put forward, and points out that they 

 cannot be of any great depth or they would not be melted 

 so quickly ; they are probably a thin sheeting corre- 

 sponding with the winter snow which extends to our own 

 mid-latitudes and quickly disappears with the approach of 

 summer. On some occasions portions of the cap were 

 temporarily hidden, and there was good reason to suppose 

 that the obscuration was produced by something of the 

 nature of clouds, though the atmosphere seems to be 

 much less dense than our own. The most striking pheno- 

 menon, however, was a projection from the edge of the 

 cap, visible at the same point in both years, which was 

 left behind as a bright strip as the cap diminished in size. 

 This is probably a range of mountains, and was found to 



