154 



MKTEOROLOGY 



to their pro|>er significance Scarcely li imjiort- 

 :inl was tin- introduction nf the hygrometer. lirst 

 matically used by I if Saiiure (died ITlf.H, 

 iinil subsequently improved I iy Iliilton, llanifll. 

 Au^ift. ami Kcgnault. From tin- period of the 

 invention "f these instruments the nnmlicr of 

 met.Mirological observers great I v iniTc..sed. ;unl a 

 large body of well .authenticated f:u-l- of real value 

 w.is oaUeeted. Tin- climates of particular parts of 

 tin- earth were approximately determined, and the 

 science made great ami rapid advances liv the in- 

 iiis into tin- lawn which regulate the 

 changes of atmospheric phenomena. 



Tin- theory f the trade winds was first pro- 

 pounded liy (ieorgc Hodley in tin- I'/ii/in-i/i/u'i-df 

 3VraM<MM for IT.'W: and it may ! mentioned as 

 a renmrkalile fact that for about half a century 

 it remained unnoticed. aiid then was iixlrpcnd 

 ently arrived at hy Dalton. The publication of 



Iialton's Meteorological K-says in I7BS, nutria an 



epoch in meteorology. Il is tin 1 first instance of 

 tin- principles of science being brought to In-ar on 

 the explanation of tin* intricate phenomena of the 

 atmosphere. Tin- idfa that vapour is an inde- 

 ]M>ndiMit clastic fluid, and that all elastic fluids, 

 whether alone- or mixed, exist independently : the 



great principles of motion of tin 1 atmosphere ; the 

 thi-ory of winds, thoir effect on the barometer, and 

 their relation to temperature and rain : observations 

 on the height of clouds, on thnmlor, nnd on meteors ; 

 Mid tlie relations of magnetism and the aurora 

 Imrealis -the-e are some of the important i|iie-ti<m* 

 discussed in thi'se remarkable essays, with singular 

 ncntcness, fullness, and hreadlh of view. 



l':ie of the most interesting and fruitful subjects 

 of ini|iiiry that engaged the attention of meteor- 

 ologists w:is ilrir. The observat ions on this subject 

 were lirst collecteil and reduced by Dr Wells, anil 

 the theory he advanced, supplemented by the 

 recent researches of Mr .lohn Aitken. ^ives a com- 

 plete explanation of the phenomenon (see DEW). 



In ls-j:t Iliiniell published his 



. 



Ess<i>/i ami t>li.irri'nliiiH.i, which, while addin 

 largely to our knowlol^e in almost every depart- 

 ment of the subject, are chielly valuable as IMMTIIIL; 



on the hvgrometry of the atmosphere. Thongu 

 the practical advantages which he nnticipateil to 

 How from it have not been realised, yet this dilli 

 cult department of meteorology still stands in- 

 ilcbte I to him perhaps more than to any other 

 phv-icist. The law of tin- dilluMon of vapour 

 through the air, its intliicncc on the haroim-tiic 

 pre-suie. and its relations to the other constituents 

 of the atmosplieie ale amoii^ the least satisfac- 

 tnily determined i|ui-tionx in meteorology. Sin,-.. 

 this element i- s,i Important in originating changes 

 of we.itber anil as an indicator nf storms, and si ...... 



BO much remains still to IN- achieved, it is in I*. 

 bo|K-d that it \\ill s,ui } more thoroughly iinc-li 



I. jiarticiilarly in its relations to solar and 

 terrestrial radiation. As the humidity to some 

 extrnt ob-tructs w ,| ar nn ,l t<>rrestrial radiation, it 

 follows that if tin- air were .piite drained of it* 

 n. pie., us vn|,,,ur the extremes i,f beat and col, I 

 would IH- so intense and insufferable that all life 

 would |MM-ih, iv< there would IH' no screen shield 

 inx tl ..... arth fiom the si ...... hin^ heat of the sun 



lav, and from the equally Hcorehinu and blight 

 ing erfecN of it- own radiation by ni^ht. 



i:ition have IK-IMI. of all iiH'Icor- 

 Obgieal ol.s.-ivatioiis. pi'iliaps tli.. least productive 



of renlt adranetno the -cji-ne.-. paitly owin^ to 



their wAiitinra<, and from the ex|H!iinc and trouble 

 attending (h"m. 



llnmlxddt s treatise on f*nl>irrni<il f.im:i (1SI7) 

 cnnntitntm a notable epoch in practical meteor 

 .'id. more recently, the present \viit-r 

 hare continued the investigation, ami given charts 



of the world, showing the temperature for each 

 month nnd for the year. In lsi>s another 

 of important idiaits were publisliiil by the writer, 

 showing, by isol>aric lines, the distribution of the 

 mass nl' the earth's atmosphere, and by arms the 

 pifvailing winds over the gloln- for the months and 

 the \ ear. These charts, since revised by him. and 

 published in one of the ('/mil' min- report*, show 

 the movements of the atmosphere and their imme- 

 diate cans... It is ihns seen thai the prevailing 

 winds are the simple result of the relative distri- 

 bution of the mass of the earths atniosjihere ; or 

 that the direction and force of the prevailing winds 

 aie simply the How of the air from a region of 

 higher towards a region of lower pie-sure, or fiom 

 where then' is surplus to where there is a deliciency 

 of air. On this broad and vital principle meteor- 

 ology rests, and it is of unhcr-.<l application 

 throughout the science in explanation not only 

 of prevailing winds, lint of all winds, and of 

 weather anil weather changes j;cnerally. Further, 

 it supplies the key to the climatologies of the 

 jjlolM' ; for climate is determined by the tempera- 

 ture and moisture of the air. and these in their 

 turn by the prevailing winds. In 18S2 I.ooniis 

 published a map representing in colours the mean 

 rainfall of the globe. This map :u:d others that 

 have been constructed for separate countries show- 

 that the rainfall is everywhere determined by the 

 prevailing winds, considered with respect to the 

 regions from which they have immediately come, 

 and the physical configuration and temperature 

 of the part of the earth's surface over which they 

 blow. The highest rainfalls are precipitated bv 

 winds which, having traversed a large breadth 

 of ocean, come up against and blow over a high 

 ridge lying across their path; and the amount is 

 still further increased if the winds pass at the 

 same time into regions the tcmpcratnre of which 

 constantly becomes colder. Of this the winter 

 rains of 'north-western Europe and the summer 

 rains of Japan are good example!. On the other 

 hand, the rainfall i~ \er\ small, or nil, where the 

 prevailing winds have not previously traversed 

 some extent of ocean, but have crossed a high 

 ridge and now advance into lower latitudes, or 

 into regions the teni|'ratiue of which is markedly 

 higher. Good examples of this are the summer 

 rains of California and adjoining regions, and those 

 of the Indus valley. 



The establishment of meteorological societies 

 during the last half itf the lllth century must 

 also be commemorated as contributing in a high 

 degree to the solid advancement of the science 

 which, more than any other, must depend on ex- 

 tensive and carefully conducted observation. A 

 special object of meteorological societies is to 



ascertain the degrees of temperature and moisture 

 in various localities, and the usual periods of their 

 occurrence, together with their ellects on the health 

 of the people and upon the different agricultural 

 products; so that, by a knowledge of the laws by 

 which the growth of such products is regulated, It 

 may be aseeitajned with Mime degree of certainly 

 whether any given article can be profitably culti- 

 vated. Hut perhaps none of the arts have benefited 

 to so largo an extent by the results arrived at by 

 meiiHirologists as navigation. The knowledge thus 

 aci|iiiied of the prevailing winds over the dill'erent 

 parts of the enrth during the different seasons of 

 the year, the regions of storms and calms, and the 

 laws of storms has both saved innumerable lives, 

 and. bv pointing out the most expeditious mill, 

 lie followed, shortened voyages to a remarkable 

 degree. In this department the name of Maury 

 (q.v.) deserves special commendation. 



Another fruit of the multiplication of meteor- 

 ological stations is the prediction of storms and 



