METEOROLOGY. 



409 



evaporation increases with increase in the projor- 

 tion of liiiitiux in the snil. and diminishes as tlie 

 -andy and coarse-grained. 



Clouds. A paper Ijy Mr. \V. Ellis on the moan 

 amount of cloud on each day of the year at the 

 Ruyal observ.r owich, on the average of 



the' fifty years ls4l-''.n.) shows that a principal 

 maximum occurs in winter and a principal mini- 

 mum in autumn, with a secondary much less pro- 

 nounced maximum in summer and a secondary 

 minimum in spring. There is. however, consider- 

 able irregularity in the succession of daily values, 

 the differences in which on consecutive days are in 

 numerous cases relatively large. Cloudless days 

 are most numerous in spring and autumn, and days 

 of little cloud are somewhat less numerous in win- 

 ter as compared with other parts of the year, while 

 days of medium cloud are much more numerous in 

 summer than in winter. 



An ingenious nepheloscope, or instrument for ob- 

 serving the clouds, has been suggested by M. L. 

 n. of the Royal Observatory. Brussels. Our 

 observations, as usually taken, are necessarily de- 

 fective because of the impossibility of establishing 

 fixed lines or points in the sky by means of which 

 the position of objects projected upon it may be 

 estimated. If. however, we could have such lines 

 or points, if we could divide the celestial calotte 

 into invariable segments, observations of nebulos- 

 ity might be made methodically and in a uniform 

 way at the different meteorological stations. By 

 turning toward different points in the horizon the 

 observer, in order to obtain the total cloudiness, 

 would have only to add the segments or parts of 

 segments covered with clouds. Such lines, though 

 they can not be found or placed in the sky, can 

 be traced on a spherical mirror, whence the part of 

 the sky visible within the compass of the horizon 

 will be reflected in all its details. 



Among the various devices for measuring the 

 heights and velocities of clouds, the observers of 

 the Meteorological Council at Kew have used pho- 

 tographic cameras fitted with theodolite mountings 

 and provided with altitude and azimuth circles. 

 Mr. Botch, at Blue Hill, Mass., measured the ve- 

 locities of clouds by timing the movements of 

 shadows cast by them' at points whose distance was 

 known. During 1890 cloud observations were made 

 in all parts of the globe under the special direction 

 of the International Meteorological Committee. 

 11 the stations used instruments, and the ma- 

 jority of those which did only employed nephelo- 

 scopes which gave the direction and apparent ve- 

 locity of the clouds by means of a mirror and 

 graduated circles. At some of the principal ob- 

 servatories theodolites and photogrammeters were 

 used. Each of the last two methods has its advan- 

 tages and disadvantages : theodolites are simpler 

 and cheaper, while photogrammeters require a cer- 

 tain amount of skill in photography. The theodo- 

 lite requires the two observers, each placed at one 

 end of the measured base to agree upon some fixed 

 point in the cloud and the precise instant at which 

 the observation should be taken, which it is not al- 

 ways easy to do, even by telephonic correspondence. 

 The calculation of the observations is subsequently 

 made from trigonometrical formulas, or by a slide 

 rule or plotting machine. With the photogram- 

 meter, which is a theodolite provided with a small 

 telescope and a camera oliscura, an agreement as 

 to the special point to be observed is not necessary. 

 It is sufficient that both observers photograph the 

 same part of the sky at the same moment. 



Mr. John Aitken brings forward a new view of the 

 manner in which ripple-marked cirrus clouds are 

 produced. The common explanation of this form 

 of cloud is that the ripple markings are due to the 



:-al movements of the air giving ri-c to a series 

 of circli-s t he ax>-> of the eddies being horizontal 

 and roughly parallel to one another. Mr. Aitken 

 believes. ,, n the other hand, that they arc clouds in 

 decay. They are gnu-rally formed out of - 

 strato-cirrus or similar cloud. When these strato- 

 cirrus clouds are observed in fine weather, it is 

 found that they frequently change to ripple-marked 

 clouds before vanishing. The pro<-i->> of tin- forma- 

 tion of these would seem to be that the strato-cirrus 

 gradually thins away till it attains such a depth 

 that if there are any eddies at its level the eddies 

 break the stratus Cloud up into parallel or nearly 

 parallel masses, while the clear air is drawn in be- 

 tween the eddies. It will be observed that th 

 planation requires the eddies, but not to produce 

 the clouding, only to explain the breaking up of 

 the uniform cirrus cloud into ripple cirrus. The 

 hypothesis is supported by the fact that lenticular 

 cirrus clouds are frequently observed with ripple 

 markings on one or more sides of them, just where 

 the cloud is thin enough to be broken through by 

 the eddies. If we watch these lenticular clouds 

 under these conditions, we frequently see the ripple 

 markings getting nearer and nearer the center as 

 the cloud decays : and at last, when nearly dis- 

 solved, the ripple markings will be seen extending 

 quite across the cloud. 



A new classification of clouds is proposed by Mr. 

 Aitken into clouds in the process of formation 

 and clouds in the process of decay. The cumulus 

 clouds are taken as an example of the former class, 

 and the nimbus of the latter. The author's obser- 

 vations on the clouds themselves have shown that 

 there is a difference in structure of these two el; 

 In clouds in formation the water particles are much 

 smaller and far more numerous than in clouds in 

 decay: and while the particles in clouds in decay 

 are large enough to be seen with the unaided eye 

 when they fall on a properly lighted micrometer, 

 they are so small in clouds in formation that, if the 

 condensation is taking place rapidly, the particles 

 can not be seen without the aid of a lens of consid- 

 erable magnifying power. In the former case the 

 number of particles falling per square millimetre is 

 small, while in the latter they are so numerous that 

 it is impossible to count them. 



Precipitation. It is generally considered that 

 near the equator the rains are everywhere heavy and 

 of nearly daily occurrence. Dr. A. "Woeikoff shows 

 in a paper on the rainfull of the Malay Archipelago 

 that in many localities in the open sea. for exam- 

 ple this is not the case. In the region in question 

 some of the wettest and some of the driest stations 

 lie within IV north and 1 south of the equator. 

 The most rain falls on the west coast of Sumatra; 

 the more level eastern Sumatra and western Borneo 

 have less rain and also less contrasts. On the north- 

 east peninsula of Celebes the rains are comparatively 

 light, and there is a well-marked dry season. In 

 Java the rainfall is lightest in the east, and the dry 

 D is longer and more sharply defined, so that 

 ition has a time of arrest corresponding to our 

 winter. 



Records of the relative fall of rain in the day and 

 night, kept for ten years at Seveiioaks. England, by 

 Mr. \V. W. Wagstaff, show that the mean annual 

 rainfall for the day was 4d inches and for the night 

 60 inches: that in winter the nights are much wet- 

 ter than the days, and are also wetter in spring and 

 summer, but with less marked difference, which is 

 least in summer. 



The white vapors or light fogs which form over 

 meadows and in narrow valleys after sunset on 

 bright summer days are ascribed to condensation 

 from evening radiation of the vapors that saturate 

 the air near the ground. A phenomenon of an op- 



