February 2, 1922] 



NATURE 



M3 



will be 2700 A.U. If, however, the particles 

 have diameters of only 1600 A.U. and unit density 

 the light pressure will be nineteen times greater than 

 the gravitation attraction. For sizes still smaller 

 the light pressure would decrease again, and for 

 diameters less than 500 A.U. gravity would once 

 more preponderate. 



If, then, the solar eruptions drive up into colder 

 regions vapours which are condensed to liquid or 

 solid particles, a sorting action will at once come 

 into play. Particles above a certain diameter will 

 be drawn back into the sun. Particles below a 

 certain diameter will be repelled away with great 

 force by light pressure, and particles of a certain i 

 critical diameter will remain suspended in space. 

 The solar corona may perhaps be in part composed | 

 of solar dust of this critical diameter, as Arrhenius | 

 has suggested. Now, as regards that dust which is | 

 repelled by the sun, it is easy to calculate the time ; 

 particles of certain sizes will take to travel to the | 

 earth's orbit and the velocities they will then possess. ' 

 Taking the particles to have unit density and three | 

 sizes, viz. 1600, 5000, and 10,000 A.U., and j 

 to be projected from the sun with velocities of | 

 200 km. per sec, I find that the times \ 

 required to travel to the earth's orbit will be respec- \ 

 tively twenty-two hours, forty-two hours, and 

 seventy-six hours. The velocities with which they 

 will arrive will be 1700 km. per sec, 780 km. per [ 

 sec, and 350 km. per sec. respectively. j 



These minute particles, composed, it may be, | 

 of carbon from the photosphere or metallic dust ' 

 from the reversing layer or volcanic ash or other 

 solar materials will in general carry electric charges, j 

 The high temperature will cause emission of elec- 



trons from the metallic particles, as also will the 

 fierce ultra-violet radiation to which they are ex- 

 posed. The metallic vapours will also be in a state 

 of ionisation, and the free electrons emitted will con- 

 dense round them gases or vapours from the chromo- 

 sphere as they pass through it. Hence the particles 

 which are repelled by light may be either positively 

 or negatively electrified or neutral. Owing to the 

 greater tendency of negative electrons to condense 

 vapours and attach themselves to groups of mole- 

 cules, the negatively charged particles may be less 

 dense and smaller than those positively charged. 

 It should be noted, however, that isolated molecules 

 or electrons are far too small in diameter to be 

 repelled by light. It is only groups of molecules 

 of at least 500 A.U. in diameter which can be re- 

 pelled. Hence these dust particles will travel out- 

 wards from the sun with very different velocities. 

 Some will come with great velocity and others with 

 small speed. 



In short, we may say that the sun, like a good 

 housemaid, dislikes dust, especially dust of a certain 

 degree of fineness, and pushes it away from it with 

 great force. The moment that this electrified dust 

 enters the earth's magnetic field with high velocity 

 forces will be brought to bear on it tending to 

 separate the negatively and the positively charged 

 particles. If H is the magnetic force of the earth 

 and V the particle's velocity, and e its charge, then 

 the separating force is Yiev where H is that com- 

 ponent of magnetic force at right angles to the 

 direction of v and the separating force is also at 

 right angles to the plane of H and v. 



(To he continued.) 



Obituary. 



Sir Ernest Shackleton, C.V.O. 



THE sudden death of Sir Ernest Shackleton on 

 board the Quest at South Georgia on 

 January 5 stopped the career of the most brilliant 

 of AntaiCtic explorers just on the threshold of the 

 South Polar regions which he was entering for the 

 fifth time with his third expedition. That such a 

 courageous and indomitable explorer should die a 

 natural death after a lifetime of hair-breadth 

 escapes from perils of ice, of starvation, of ship- 

 wreck, and of war is a grim stroke of Nature's 

 irony. Great as his loss is to geographical ex- 

 ploration, we cannot but recognise his end as happy, 

 for his life was arrested in the full course of the 

 enthusiastic pursuit of a great and crowning adven- 

 ture. The sympathy of all who appreciate high- 

 hearted deeds will flow towards his wife, to whose 

 co-operation much of his success was due ; tOAvards 

 his shipmates, who have nobly resolved to carry on 

 the voyage; and towards Mr. John Q. Rowett, 

 whose friendship for Shackleton made him under- 

 take the main financial burden of the expedition. 



Ernest Henry Shackleton was born at Kilkee, in 

 Ireland, in 1874, removed to London with his father 

 while still a schoolboy, and at an early age insisted 

 NO. 2727, VOL. 109] 



on going to sea instead of following his father's 

 profession of medicine. After voyages to South 

 America and other parts of the world, he entered 

 the service of the Union Castle Co., where he was 

 during the stirring days of the Boer War. He had 

 become an ofiicer of the Royal Naval Reserve before 

 the plans of the Antarctic expedition on the Dis- 

 covery fired him with the desire for exploration. His 

 application for a post on the expedition was refused, 

 persisted in, and finally accepted, and he had a 

 strenuous time on board as junior watch-keeping 

 officer. The expedition sailed in August, 1901, and 

 from the outset Shackleton was eager to undertake 

 every piece of voluntary work. He assisted in the 

 chemical and oceanographical observations, assumed 

 the editorship of the South Polar Times, and read 

 up the history of polar exploration. When Capt. 

 Scott was making up his party for the great southern 

 journey of 1902-3 he included Shackleton, who 

 thus took part in establishing the " farthest south " 

 of lat. 82° 17' S., and saw the great range of 

 mountains bordering the Ice Barrier on the west 

 and stretching far to the southward. On the return 

 journey Shackleton broke down from an illness 

 which was probably scurvy, but he struggled on to 



