9 2 ASTRONOMY 



close resemblance to the filaments, or possibly (as Deslandres suggested later) to the 

 alignments. 



We may confidently expect more light on these important points from the magnifi- 

 cent " tower telescope " which G. E. Hale has set up on Mount Wilson, in California. 

 The tower itself (176 feet from ground to top of dome) was already erected in September 

 1910 when the International Union for Solar Research assembled on Mount Wilson; 

 but there has been some delay in obtaining a suitable object-glass for the telescope (of 

 12 inches aperture and 150 feet focal length). Success was not attained till the third 

 attempt, but a very satisfactory lens was delivered by J. Brashear in May 1912: and the 

 latest reports indicate that the chief difficulties of this immense undertaking have been 

 overcome. It was feared by various critics that so high a tower would shake, but 

 Hale's ingenious plan of encasing an inner tower of rods in an outer tower of tubes (to 

 each rod a tube surrounding it) has proved perfectly successful, so that " the delicate 

 operation of photographing the flash spectrum has been carried out when the wind was 

 blowing twenty miles an hour, and there has never yet been an occasion when it was 

 necessary to stop work because of trembling of the image." 



The sun's rays are reflected down this huge fixed telescope by moving mirrors at 

 the top of the tower, and an unexpected difficulty declared itself in the unequal heating 

 of these mirrors, with consequent changes in focal length of several feet. It was met 

 and defeated by keeping water steadily circulating in water jackets covering the edges 

 and backs of the mirrors. 



At the foot of the tower the rays are received into a well 75 feet deep, containing a 

 combined spectrograph and spectroheliograph. A revolving table makes it possible to 

 change the apparatus in the well with ease and rapidity, so as to obtain within a few 

 minutes half-a-dozen spectroheliograms in different colours, and then a number of 

 spectrograms. 



Many solar phenomena centre round the solar spots, as to the origin and nature of 

 which we are still but ill informed. The most striking feature of their appearance is the 

 cycle of about 11.125 years; but A. Schuster showed recently that this is not the only 

 cycle concerned, periods of 4.8, 8.3 etc. years being also traceable in the records. His 

 work suggests a major period of 33.375 years of which others are submultiples: thus 

 dividing by 7 we get 4.77 years, a period found by Schuster in his analysis, and confirmed 

 by a special small rise in the last cycle. The period of 33.375 years is close to that of 

 the November meteors, and it is possible that the revolution of the swarm may exercise 

 control over the occurrence of sunspots. Schuster has also examined the influence of 

 the planets, obtaining positive evidence which satisfies him, though it has been chal- 

 lenged by F. J. M. Stratton working on somewhat different lines. Schuster remarks that 

 though the tidal attraction due to a planet would be very small compared with the sun's 

 gravitational attraction the latter may be effectively diminished by light repulsion for 

 very small particles near the surface; in which case the tidal action of the planet might 

 easily assert itself. 



A summary of much recent work on the sun (especially by Hale who detected the 

 magnetic phenomena through the Zeeman effect, and by Evershed who detected the 

 radial movements in sunspots) is provided by the hypothesis as to the nature of a sun- 

 spot recently formulated by Hale, as follows: 



"As the result of an eruption or some other cause tending to produce rapid convection, 

 a gaseous column moves upwards from within the sun toward the surface of the photosphere. 

 Vortex motion is initiated by the difference in rotational velocity of adjoining zones, or by 

 irregularities of structure, and is maintained by convection. The circulation in the vortex 

 is vertically upward and then outward along the photosphere, as in a terrestrial tornado. 

 Expansion produces cooling of the centre of the vortex, and a comparatively dark cloud 

 (the umbra) results. As in Marker's electric furnace experiments, 1 a rapid flow of negative 

 ions sets in towards the cooler gases at the centre from the hotter gases without. The ions, 

 whirled in the vortex, produce a magnetic field. The descending gases (specially hydrogen) 

 in the higher atmosphere of the sun are drawn in toward the pole of the magnet along the 

 lines of force, as in Birkeland and Stormer's theory of the aurora. This accounts for the 

 configuration of the hydrogen (Ha) flocculi, as shown by the spectroheliograph. 



