178 



NATURE 



{Dec. 26, 1878 



in the far east, where, as near Khiva, a summer of more than 

 tropical heat is succeeded by a winter of Arctic rigour. In a 

 very extreme degree the climate of Astrakhan contrasts with that 

 of Fuegia, and yet the mean temperature of the two is about 

 the same ; but in the one the seasons are excessive, in the other 

 the difference is but small. The difference in the products of 

 the two countries is thus very great : on the one hand, plants 

 requiring great heat, but able to withstand the cold, on the other 

 plants of a more tender nature which can flourish with a very 

 moderate amount of warmth ; in the one grapes and corn, in the 

 other fuchsias and veronicas. 



In studying climate it is therefore necessary to observe not 

 only the greatest heat and the greatest cold, but also the mean 

 temperature. These can only be observed by means of thermo- 

 meters, for personal feelings may be the effects of many other 

 causes — of wind or evaporation, or state of health, or pecu- 

 liarity of constitution, and are absolutely no index to the state of 

 the air temperature. 



The lecturer then proceeded to speak of the different kinds of 

 thermometers, several of which were exhibited, and of the 

 several stands for sheltering them. The Meteorological Society 

 . has decided positively in favour of the Stevenson stand, and 

 directs its observers to record the temperature at 9 a.m. and 

 9 P.M., as well as the highest and lowest, as registered by the 

 maximum and minimum thermometers. He then described some 

 novel and ingenious contrivances for automatic registering, such 

 as the "turnover" of Messrs. Negretti and Zambra, and the 

 " chronothermometer " of Mr. Stanley, and concluded by point- 

 ing out that these instruments were but a means to an end, and 

 Ihat the study of climate was the study of nature in one of her 

 most beautiful and most varied aspects. 



Mr. Ley's lecture was on 



Clouds and Weather Signs 



The lecturer dwelt, in the first place, on the unsatisfactory 

 ■condition of this portion of meteorology, as contrasted with 

 those branches of the science in which instruments are em- 

 ployed. The great impediment to our progress arises from 

 the fact that cloud-observation is, in large measure, an in- 

 <:ommunicable art, requiring a special training of the eye. 

 Specimens of the objects of study cannot be exhibited, neither 

 is it possible, in illustration of the subject, to refer to types 

 of cloud depicted in the well-known paintings of the best 

 artists, because the latter, aiming at the production of atmo- 

 spheric effects, employ the materials most easy to handle, which 

 are commonly the least typical cloud-forms. The old classifica- 

 tion and nomenclature of clouds is highly unsatisfactory, having 

 been framed at a time when the relation of wind and weather to 

 the distribution of barometric pressures was unknown ; and with 

 this relation the forms and movements of the clouds are inti- 

 mately connected. 



As regards configuration, clouds seem naturally divisible into 

 two groups, those which arrange themselves in layers, whose 

 vertical diameter is small as compared with its horizontal, and 

 those which assume spherical or hemispherical shapes ; and this 

 division is related to certain physical conditions of the atmo- 

 sphere and of the earth's surface beneath the cloud. It is, how- 

 ever, essential that we should possess some name or system of 

 names to distinguish those clouds which are conveyed by the 

 upper currents, and the term cirrus, with its compounds, must 

 be more closely restricted to this class of clouds than has yet 

 been done. From the use of weather-maps a new science of the 

 winds has originated, on which all attempts at weather fore- 

 casting must be based. The movements of the upper clouds 

 afford most valuable information concerning the distribution and 

 movement of the areas of high and low barometric pressure. 

 Rules by which this information may be interpreted, based in 

 great measure on a former investigation by the lecturer,^ are 

 somewhat complex, and cannot well be given in a brief resumS 

 like the present.'^ It may be sufficient to explain that in the front 

 of an advancing barometric depression there usually extends a 

 bank of the halo- producing cirro-stratus, the exterior edge of 

 which is, roughly speaking, a parabola, the focus of which lies in 

 the line about to be traversed by the centre of the depression. 

 On the right-hand of the centre this bank or sheet is abruptly 

 broken and is succeeded in the rear by local shower-clouds. On 

 the left-hand the sky commonly continues overcast, but the cloud- 



'" Kelation between the Upper and Under Currents of the Atmosphere 

 around Areas of Barometric Depression." Quart. Joum. of Met. Soc. 

 •rol. ill. p. 437. 



* The lectures will shortly be published. 



plane gradually descends until little is to be seen but low stratus. 

 A backing of the upper current takes place until after the centre 

 of the depression has passed. In whatever direction a depression 

 is advancing the same characteristic phenomena are observed. 

 Thus changes in the clouds indicate to us probable alterations of 

 wind and weather. 



While the nimbus, which exists in the front of a depres- 

 sion, first makes its approach evident by changes in the higher 

 cloud-strata, the process of nubification is the converse of this 

 in those local showers which commonly occur on the right-hand 

 and in the rear of a centre of depression, and therefore when 

 the barometer is rising or just about to rise. These latter are 

 developed in an upward direction through the formation of cumu- 

 lus. The precipitation which occurs in them— always preceded 

 by a change of appearance in the domes of cloud, which assume 

 a soft and cirriform aspect — is attributed to the neutralisation of 

 electricities as the summit of the cloud passes into a higher 

 region ; but there are important differences of appearance 

 between those cumuli which are likely, and those which are 

 unlikely, to undergo this transformation, 



A physical explanation is given of the ordinary weather signs 

 derived from the colours of the sky, from "visibility," and from 

 unusual refraction. Attention is invited to some peculiar types 

 of cloud, especially to a very elevated turreted stratus, often erro- 

 neously termed cirro-cumulus, occurring with high temperature 

 on the south-western borders of anti-cyclones, a forerunner of 

 thunderstorms. The formation of the low-level stratus and 

 of the fog which usually prevails in our winter anti-cyclones, 

 seems to be due to a downward movement of the air at a time 

 when the earth's surface is colder than the atmosphere at a slight 

 elevation above it. 



In the course of this lecture sketches of some of the more 

 definite varieties of cloud, with arrows indicating the direction 

 of the currents, and a diagram showing the movements of the 

 upper currents, and the prevailing cloud-types around areas of 

 depression, were exhibited by the aid of the oxy-hydrogen lime- 

 light. 



THE MOTION OF A LUMINOUS SOURCE AS 

 A TEST OF THE UNDULATORY THEORY 

 OF LIGHT 

 I. A LT HO UGH the undulatory theory of light may 

 XJL now be considered as completely established, 

 still any confirming test of a physical theory is in itself 

 interesting as a fresh illustration of a natural truth. 

 Considering how at one time crucial tests of this theory 

 were sought after, it Avould appear perhaps rather an 

 anomaly that attention should apparently not have been 

 given to the effect attendant on the motion of a luminous 

 source as a test between the two rival (undulatory and 

 corpuscular) theories, and that more especially as the 

 test would appear to be in principle a simple and de- 

 cisive one. I should have considerable diffidence in 

 directing attention to this point, but no record apparently 

 exists of experiments proposed or attempted with this 

 view. It might be said that the possible existence of 

 practical difficulties in the way of carrying out the test 

 may account for this ; but then practical difficulties are 

 seldom allowed to stand in the way, if a theoretic prin- 

 ciple be correct ; and, unless a thing were seriously pro- 

 posed and discussed, no attempt would be made to sur- 

 mount the difficulties that might exist in the way of 

 carrying it out. Sir John Herschel, as far as he touches 

 on this point, would appear to have had the idea that 

 such a test between the two theories could not, in prin- 

 ciple, be applicable ; for he says (" Outlines of Astro- 

 nomy," p. 214), speaking of the effect attendant on the 

 motion of a luminous body, "The effect in question, 

 which is independent of any theoretical views regarding 

 the nature of light, .... &c." It is true he mentions 

 afterwards, in a foot-note, a difference in the effect in 

 the case of the two theories as regards the velocity of 

 light, in the case of a luminous source moving directly 

 towards or from the observer ; but the following funda- 

 mental difference in the case of the two theories appears 

 not to hare been taken into account. 





