NATURE 



[October 17, igi; 



Observers were stationed at various distances up to 

 54 kilometres from the signals, and naked-eye records 

 were taken. There were three sets of experiments, 

 to determine (i) the relative advantages of circular and 

 Hat-wick lamps ; (2) the range of red and green lights ; 

 and (3) the distance at which two separate lights can 

 be distinguished. The results showed (i) that the 

 light from the flat wicks was considerably brighter 

 at a . distance than that from the round wicks, 

 particularly when the line of vision formed a 

 right angle with the sides of the wick, and (2) that 

 the range of the red lights (flat wicks) was approxi- 

 mately double that of the green, the latter being 

 scarcely visible at 3 kilometres, while the former were 

 still bright at 5 kilometres. In the third set of ex- 

 periments observations were taken at distances of 

 2 and 3 kilometres of pairs of lights separated by 

 intervals of 2, 3, 4, and 5 metres. At 3 kilometres 

 none of the pairs were distinguishable as separate 

 lights when the space between them was less than 

 4 metres. With a separation of 2 metres red or green 

 pairs were jnerged into one bright light. When red 

 and green lights were shown simultaneously, the 

 green light was eclipsed altogether by the red at 

 2 kilometres if the distance separating them was not 

 more than 2 metres. 



Part 15 of the Verhandlungen of the German 

 Physical Society contains a description of a mechanical 

 pump for high vacua recently devised by Dr. W. 

 Gaede, which makes use of a principle not previously 

 utilised in the construction of such apparatus. If a 

 shaft revolving in a well-fitting bearing has a circular 

 slot cut in it, the air in the slot will to a large extent 

 be carried round with the shaft. If at one part the 

 bearing projects into the slot so as to fill it completely, 

 the gas in the slot will be carried round with the shaft 

 from one side of the projection to the other, and the 

 pressure will in consequence be less on one side than 

 on the other. If two openings are made through the 

 bearing, one on each side of the projection, air will 

 be drawn in through one and delivered through the 

 other. By making a number of slots in the shaft 

 and connecting the openings into them in series, the 

 action will be intensified. A pump constructed on 

 these lines exhausts five or ten times as fast as one 

 of Dr. Gaede's well-known mercury pumps taking the 

 same power, and deals with the water vapour as well 

 as the gas, so that no drying materials are necessary. 

 As it works better at low than at high pressures 

 it is run in conjunction with another pump, which 

 reduces the pressure to a few centimetres of mercury. 



In a publication of the R. Accad. delle Sci. dell' Inst. 

 di Bologna, which has recently come to hand, Prof. 

 .\. Righi describes some interesting e.xperiments on 

 the emission of ions in directions perpendicular to 

 that in which the main discharge passes. Two wire 

 electrodes are sealed into a cylindrical vacuum tube, 

 perpendicular to the axis, with only their points ex- 

 posed. When an impulsive discharge is passed be- 

 tween them it is found that ions are shot along the 

 axis of the tube. The distance they penetrate and 

 their relative numbers under different conditions are 

 studied by collecting them in a suitably placed Fara- 

 NO. 2242, VOL. go] 



day cylinder. By an ingenious arrangement of vanes, 

 which rotate when the ions strike them, it is possible 

 to follow the paths of the particles. This transverse 

 emission of ions is most vigorous near the ends of 

 the main discharge. When the tube is placed in a 

 magnetic field parallel with its axis the neutral 

 doublets, already investigated by Prof. Righi in earlier 

 papers, are formed, and, as would be expected, the 

 Faraday cylinder collects less charge. On the other 

 hand, owing to the large mass of the doublets, the 

 mechanical effects are increased. 



An article in Engineering for October 11 recalls the 

 discussion of a few years ago on the distribution of 

 shearing stresses on the horizontal layers of a dam. 

 Messrs. Wilson and Gore showed experimentally in 

 1908 that the stresses did not follow a parabolic distri- 

 bution, but were much more uniform. Prof. E. G. 

 Coker has lately described experiments at the Royal 

 Society on thin celluloid sheets under shearing stress, 

 the conditions resembling that of the web plate of a 

 plate girder. It has been contended that the shearing 

 stresses in the girder web follow the parabolic law, 

 but these experiments indicate that this law is only 

 approached when the plate is shallow. Otherwise the 

 shear curve had no maximum at the centre of the 

 specimen, but showed two equal maxima, which are 

 at points situated at a distance from the ends equal 

 to rather less than the width of plate under test. 

 Reducing the depth of the plate causes these two 

 maxima to approach each other, and they finally 

 coalesce when the depth of the specimen is about 

 equal to its width. 



A SECOND edition of Mr. T. H. Byrom's " Physics 

 and Chemistrv of Mining : an Elementary Class-book 

 for the Use of Mining Students," has been published 

 by Messrs. Crosby Lockwood and Son. In this edition 

 the chapter on magnetism and electricity has been 

 omitted, and additional matter has been introduced in 

 both the physical and chemical sections. The price 

 of the volume is 3s. 6d. net. 



Messrs. H. F. .\ngus and Co., of Wigmore Street, 

 London, have issued a new catalogue of second-hand 

 scientific apparatus and accessories which are avail- 

 able for sale, exchange, or hire. We notice that all 

 the instruments listed, unless otherwise stated, have 

 been tested, adjusted where necessary, and are capable 

 of work of equal precision as when new. Interesting 

 particulars are given in the list of microscopes and 

 accessories, as well as of various other optical instru- 

 ments. 



OUR ASTRONOMICAL COLUMN. 

 Gale's Comet, 19120. — London urban skies have 

 remained "comet-proof for some time now, but Mr. 

 Franks, writing to The Times (October 15), reports 

 that he saw Gale's comet very well, with a 6-inch 

 refractor, during the week ending October 11, at East 

 Grinstead. He states that it appeared to be brighten- 

 ing, for it was about fifth magnitude when he first 

 saw it, and was nearer fourth on October 11. On 

 this date it was a fine object, plainly seen in the 

 finder, and, by sighting along the telescope, it could 

 be seen bv the naked eye as a misty spot about half a 



