METEOROLOGY. 



439 



velocity of all clouds is greater in winter than in 

 summer. 



It has been found by 0. T. R. Nilson that clouds 

 may be produced by the action of ultraviolet light 

 on moist air. When the light from an arc-lamp 

 was brought to a focus by means of a quartz lens 

 within a vessel containing moist, dust-free air, a 

 bluish fog became visible in the course of a few 

 minutes along the path of the light. The cloud 

 particles remained in suspension for hours after 

 the light was cut off. The phenomenon was shown 

 even in unsaturated air, but the faint blue haze which 

 then developed took much longer to form. When 

 the radiation was not sufficiently intense to show 

 these effects, a dense fog could still be obtained by 

 bringing about slight su pel-saturation by expan- 

 sion. The clouds, unlike those obtained by Tyndnll 

 and by Aitken by the action of light on various 

 vapors, are due to the ultraviolet rays above ; for 

 it' a thin sheet of glass or mica (substances which 

 are opaque to those rays) be interposed, not a trace 

 of fog or rain is formed, even when a high degree 

 of supersaturation is brought about by expansion. 

 It is possible that the small particles to which the 

 blue of the sky is due are the result of this action 

 of the ultraviolet rays, of which sunlight, when it 

 first enters our atmosphere, doubtless contains a 

 plentiful supply. 



In connection with the attempts to establish a 

 twenty-six day period for meteorological phenomena. 

 Prof. Arthur Schuster is cited as being led to think, 

 from a critical examination of the published investi- 

 gations on the subject, that " although the magnetic 

 phenomena and the occurrence of thunderstorms 

 seem to be affected by a period of twenty-six days 

 and of its first multiple, the subject requires a good 

 deal of further study before we can be sure as to 

 the exact nature of the period. Even though it 

 may be considered as proved, it must not necessarily 

 be assumed that it is due to solar action. If it is a 

 question merely of magnetic disturbances, there 

 does not seem to be any great improbability, how- 

 ever, that some periodicity may be connected with 

 the sun's rotation about its axis, especially at times 

 of great sun-spot activity." 



Winds. Addressing the Royal Meteorological So- 

 ciety upon the progressive movements of the cores of 

 the permanent high-pressure areas which arc found 

 to be associated with different localities at different 

 times of the year, Major H. E. Rawson referred 

 to previous investigations by Abercromby, Scott, 

 Loomis, H. C. Russell, and Buchan, and then pro- 

 ceeded to give the results of an examination which 

 he had made of all the available synoptic weather 

 charts for the eleven years 1881 to 1891. During 

 this period there were 212 cases in which the center 

 or core of an anticyclonic system was over the 

 British Isles, and of these 130"were due to the At- 

 lantic system, 41 to the Scandinavian, 17 to the 

 Greenland system, 22 to the Atlantic and Scandi- 

 navian systems extending and merging together, 

 and 2 to the same thing occurring in the case of 

 the Atlantic and Greenland systems. It is thus evi- 

 dent that the greater number of the British cyclones 

 are owing to the Atlantic system. They occur in 

 all months, but more especially in January, June, 

 and October, and are least frequent in April and 

 November. When such cyclones move away from 

 I lie British area the direction is very much in- 

 fluenced by the season of the year; by far the 

 largest number drift off in some direction between 

 northeast, through east to south, and take the more 

 southerly course in December, January, and Feb- 

 ruary. A few between April and July move west 

 or southwest, and still fewer north or northwest. 



The report of the Wind Force Committee of the 

 Royal Meteorological Society giving the results of 



experiments with anemometers attached to the rig- 

 ging of ships at different heights, finds as general 

 facts deducible from them, that the instruments 

 must have a fairly clear exposure to be of much 

 value, and that for a mile at least all around there 

 should be no hills or anything higher than the posi- 

 tion they occupy ; that on a ship the results may 

 be considered fairly accurately determined by hav- 

 ing the. instrument 50 feet above the hull, but that 

 on land it will generally be necessary to carry the 

 instruments somewhat higher, the height to be 

 determined by the local conditions; that no other 

 form of anemometer offers such advantages as the 

 pressure tube, from the fact that it can be run up 

 and secured easily at the desired height above a 

 building, and that the pipes and stays can be slight 

 so as to offer no resistance to the wind and cause 

 no deflecting currents. The committee found that 

 the ship itself modified the indications of the lower 

 anemometers, while some low hills and trees a 

 quarter of a mile away affected the wind velocity 

 from the quarters in which they lay. 



In the instrument of Prof. F. E. Nipher for 

 measuring wind pressure, two equal thin metal 

 disks, 2-J- inches in diameter, having beveled rims, 

 are screwed together so as to leave a small space 

 between, into which a connecting tube is passed 

 through the center of one of the disks. The end of 

 the tube is flush with the inner surface of the disk, 

 and the interspace is filled up with a certain n\im- 

 ber of layers of wire screen which project at least 

 half an inch beyond the edges of the metal disks. 

 When this simple device is placed in a stream of 

 air, it is found that the effects of rarefaction and 

 compression, set up at different psirts of the porous 

 screen, neutralize each other, so that the pressure 

 at the mouth of the tube is the same as the true 

 intrinsic pressure of the external air. This prop- 

 erty of the collector was severely tested by thrust- 

 ing it out of a carriage window in a train which 

 was traveling at the rate of sixty miles an hour. 

 No effect on the gauge could be noticed, although 

 the instrument was sufficiently tangent to the edge. 

 The gauge employed was a water manometer con- 

 sisting of a cylindrical vessel partly filled with 

 water, with a straight glass tube lead'ing out from 

 the bottom and inclined at 5 in 100 to the hori- 

 zontal. The open end of this tube was in commu- 

 nication with a collector of the form suggested by 

 Abbe so as to secure a standard pressure of com- 

 parison. 



From observations of dust particles in the at- 

 mosphere made at Mont Saleve in Savoy, the oasis 

 of Biskra, the village of Tortola and the forest of 

 Loimola in Finland, the neighborhood of Cristian- 

 sund on the west coast of Norway, and the island 

 of Grip, M. G. Melander finds that the number of 

 particles generally increases with the dryness of the 

 air. This fact seems adequate to account for the 

 influence of the direction of the wind that has been 

 observed at different stations. Aitk'en's theory 

 that the number of dust particles diminishes as the 

 wind increases in strength seems to be liable to 

 modification in the light of these observations and 

 may be regarded as subject to local conditions. 

 M. Melander further finds that the vapors from a 

 saline solution carry, even at the ordinary temper- 

 ature, particles of salt in suspension or solution ; 

 the number of particles in this case increases in the 

 vicinity of the liquid. He concludes that, at least 

 in many cases, particles of salt suspended in the at- 

 mosphere are causes of fogs, clouds, and rain. This 

 hypothesis is confirmed by the deposits left by the 

 evaporation of raindrops on a plate of gl'ass. There 

 seems to be an infinite number of saline particles 

 constantly in the atmosphere which when dry are 

 almost without action, are light and constantly in- 



