5IO 



NATURE 



[December 15, 192 1 



■square miles, is divided into sixteen rainfall districts 

 and has 151 observing stations. Matabeleland, with 

 an area of 60,728 square miles, is divided into thirteen 

 rainfall districts with a total of 88 observing stations. 

 The average seasonal, or annual, rainfall, ending 

 June, for the whole of Southern Rhodesia is 3038 in., 

 that for Mashonaland 3407 in., and for Matabele- 

 land 2669 in. The heavier rainfall in Mashonaland is 

 accounted for by its being closer to the sea coast, 

 -where it has more favourable opportunity of catching 

 the precipitation from the moisture-laden winds which 

 blow from the eastward. The average amounts of 

 rain for the several districts given in the map do not 

 appear very divergent, but the falls in the several 

 seasons are said to be sometimes very different. 



In No. 17 of the Geophysical Memoirs of the 

 Meteorological Office Dr. Chree discusses "Simul- 

 taneous Values of Magnetic Declination at Different 

 British Stations." Of recent years the Meteorological 

 Office has issued two-hourly readings of the magnetic 

 declination at Kew, which are published during the 

 ensuing week for the benefit of mining engineers and 

 surveyors. The question has arisen as to how far 

 the values and changes thus recorded are applicable 

 as standards at distant stations in the British Isles. 

 In order to throw light on the subject, Dr. Chree has 

 made a large number of detailed comparisons between 

 the daily magnetic changes at Kew, Falmouth, Esk- 

 dalemuir, and Stonyhurst. It appears that the irregu- 

 lar movements in declination recorded at any two 

 British stations show a general similarity, and certain 

 general relationships of a not very definite kind are 

 found between the variations at the different observa- 

 tories. There are, however, differences which are of 

 importance where accuracy in surveying is required 

 to within 10' of arc. In any case, surveyors' observa- 

 tions taken during a time of short-period oscillations, 

 as indicated by reference to observatory records', 

 should be repeated. 



In the November issue of the Dutch periodical, de 

 Natuur. Mr. J. W. Giltay, of Delft, describes an in- 

 strument, the "optaphe," intended to enable blind 

 persons who cannot use the optophone (owing to their 

 not having an ear for musical differences), to read 

 common printed matter by feeling. Five selenium 

 cells are mounted on a screen, and the enlarged image 

 of the letter to be read slides over the cells. Each 

 cell is connected with a relay and a battery. So long 

 as the cell is illuminated, the armature is drawn to- 

 wards the electromagnet, and a contact is open. As 

 soon as the cell, or part of it, is darkened by a part 

 of the (black) image of a letter, the magnet lets go 

 the armature, and the contact is closed. This' con- 

 tact shuts the secondary of a small induction-coil, in 

 the primary of which an interrupted current is cir- 

 culating. A box receiver is connected with the 

 secondary; so long as this secondary is shut by the 

 relay the diaphragm vibrates. The tips of the five 

 fingers of, say, the right hand are touching lightly on 

 the diaphragms of five receivers. The combinations 

 of the vibrations of those five telephones play the 



NO. 2720, VOL. 108] 



same rdle as the five tones of the optophone. The 

 diaphragms are not circular; part of them has been 

 cut off, as the usual diaphragm of a small receiver 

 does not give sufficiently strong vibrations. For 

 further particulars and illustrations reference should 

 be made to the Dutch periodical. 



A NEW light alloy bearing the name of " Silumin " 

 has been placed on the market in Germany. Accord- 

 ing to the Zeitschrift des Vereines deutscher In- 

 genieure of November 5, this alloy, which was exhi- 

 bited at the recent motor show in Berlin, contains 

 14 per cent, silicon and the remainder is aluminium. 

 It has a specific gravity of 2-5-2-65, i.e. 10 per cent, 

 lower than the usual alloys of aluminium-copper- 

 zinc. The tensile strength is 12-7 tons per sq. in., 

 which, is 25-30 per cent, higher than that of the 

 alloys mentioned, while the elongation (5-10 per 

 cent.) is double that of the common alloys. The 

 tensile strength decreases slowly with rise in tem- 

 perature. At room temperature, silumin has a 

 hardness figure of 60 kg. per sq. mm. (38 tons per 

 sq. in.) with a load of 500 kg. and a 10 mm. ball, 

 and at 350° C. 20-25 kg. per sq. mm. (i2-6-i5-7 ^^^"^ 

 per sq. in.). It remains impervious to saturated 

 steam, while dilute (25 per cent.) nitric acid, and even 

 concentrated acid do not attack it as much as they do 

 pure aluminium. In the presence of other acids and 

 alkalies, it behaves much the same as pure aluminium. 

 The thermal conductivity of silumin is to that of pure 

 aluminium as 4 : 47, while the thermal expansion 

 coefficient is o-88, taking that of pure aluminium as i. 

 Silumin is produced by combining the two con- 

 stituents, with certain additions, but it can also be 

 manufactured electrolytically in the same way as pure 

 aluminium. After manufacture it is "refined" by 

 appropriate treatment. 



A PAPER read by Dr. James W. French before the 

 Optical Society on November 10 contained data re- 

 lating to the interocular distance of 409 individuals 

 tested. For men over eighteen years of age the 

 average interocular distance recorded was 63 mm. ; 

 the smallest value was 56 mm., and the largest 72 mm. 

 For women the mean value was 61' mm., i.e. 2 mm. 

 less than the average interocular distance of the men. 

 The smallest separation recorded was 54 mm., 2 mm. 

 smaller than the smallest male value. The maximum 

 separation was 675 mm., i.e. 45 mm. smaller 

 than the highest male value. For adults of both sexes 

 the average interocular distance was 62 mm. After 

 seventeen years of age there does not seem to be any 

 definite change in the average interocular distance of 

 male adults. At fifteen the average was found to be 

 596 mm., and at sixteen, 61-4 mm. From these 

 measurements it is seen that binocular instruments 

 adjustable between the limits of 56 mm. and 72 mm. 

 would suit most users. Prismatic binoculars are 

 usually made adjustable between the limits of 57 and 

 70 mm. This higher limit of 70 only excludes about 

 ^ per cent, of the total number of individuals tested. 

 The lower limit of 57 excludes a larger proportion, 

 namely i^ per cent., the total excluded by these ex- 

 treme limits being, therefore, only about if per cent. 



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