Feb. II, 1886] 



NATURE 



atoms double-linked, others two pair, and another as many as 

 three pair. On these three suppositions its refraction equivalent 

 would be 73-0, 75 '2, and 77 '4 respectively. Now the various 

 isomeric terpenes have a refraction equivalent of about 72 'Qj 

 leading to the belief that only one pair of carbon atoms is in 

 that condition. This conclusion is enforced by the specific 

 dispersion, which averages 0^0299. From the analogy of other 

 compounds containing ten atoms of carbon and which are of 

 known constitution, about this amount of dispersion might be 

 expected to occur in a CioHnj having one pair of carbon atoms 

 double-linked. 



I do not know how far chemists may be disposed to accept 

 these optical properties as the arbiter between rival theories of 

 constitution ; but their value as helps will not be denied. 



With reference to this mode of experimenting, it should be 

 borne in mind that a very small quantity of a substance suffices 

 for the purpose of observation ; and whereas chemical processes 

 use up material, the refraction of a liquid can be taken with no 

 other loss than the small amount which unavoidably clings to 

 the vessels employed. This even may be recovered if it is 

 worth the trouble. 



A table of the approximate refraction erjuivalents of forty-six 

 of the elements was drawn up for my paper in the /'////. Trans. 

 of 1869. Many of these have since been re-determined, either 

 by myself or other observers, and a new edition of the table was 

 prepared for the Montreal Meeting of the British Association, 

 and appears in the American Journal of Science of January 

 1885. Carbon, oxygen, nitrogen, sulphur, phosphorus, iron, 

 chrrniium, silicon, and doubtless other elements have two or 

 three different v.ilues ; and the special circumstances under 

 which these dilTerent effects upon the r.ays of light occur offers 

 a most promising field for any future investigator. 



T. H. Gladstone 



THE RAINFALL OF THE BRITISH ISLANDS'" 

 /"""LIMATE may be defined as that peculiar state of the 



— atmosphere in regard to heat, moisture, and rainfall which 

 prevails in any particular place, together with its meteorological 

 conditions generally, in so far as these influence animal and 

 vegetable life. The diversified characters which climate displays 

 may be referred chiefly to the combined operation of these four 

 different causes, viz., distance from the equator, height above the 

 sea, distance from the sea, and prevailing winds. 



The greatest differences, however, in the local climates of 

 places situated at no great distance from each other arise from 

 differences in the rainfall. The arid plains of the North- 

 Western Provinces of India as compared with the fertile higher 

 slopes of the Himalayas contiguous to them, and the widely 

 contrasted climates of the western and eastern slopes of Scan- 

 dinavia respectively, may be cited as illustrations. In the 

 British Islands there are perhaps no stronger contrasts of climate 

 than those pre ented by Skye and the Laigh of Moray. The 

 mean temperature of these two regions in no month of the year 

 differ so much as two degrees, and for several of the months 

 they are nearly identical. But the rainfall of Skye rises towards, 

 and in many places exceeds, 100 inches annually, whereas over 

 the Laigh of Moray it is only about 26 inches. Now it is this 

 difference in the rainfall, with the clear skies and strong sun- 

 shine that accompany it, which on the one hand renders the 

 south shores of the Moray Firth one of the earliest and finest 

 grain-producing districts of .Scotland ; and, on the other, renders 

 the island of Skye quite unsuitable for tlie remunerative cultiva- 

 tion of cereal crops. It is this aspect of the rainfall which gives 

 it so paramount a place in the climatology of a country. 



Of all meteorological data, the rainfall is the most difficult to 

 represent cartographically ; and there is no other way to arrive 

 at even a tolerable approximation to the average rainfall of a 

 district than by numerous rain-observing stations well distributed 

 over its surface. Hence in this inquiry al! available statistics of 

 the rainfall for the period of years selected have been used, — the 

 number of stations being loSo in England and Wales, 547 in 

 Scotland, and'2l3 in Ireland — in all, 1S40 stations. Notwith- 

 standing this comparatively large number of rain-gauges, very 

 extensive districts remain wholly, or all but wholly, un- 

 represented. 



The period selected for the investigation is the twenty-four 



* An Address delivered to the Philosophical Society of Gl.isgow on 

 December 16, 1S85, at the request of the Council, by Ale.xander Buchan. 



years ending 1883, and the principal sources from which the in- 

 formation has been obtained are the returns published by the 

 Meteorological Societies of England and Scotland and by Mr. 

 Symons. For the method of discussing the results we refer to 

 the recently published Part of the Transactions of the Scottish 

 Meteorological Society, pp. 131-33. It may be here enough to 

 say that the whole of the averages have been calculated for, or 

 reduced to, the same term of twenty-four years beginning with 

 i860 and ending with 1883. 



The 1840 averages were then transferred to large maps of 

 England, Scotland, and Ireland, and from the results thus 

 shown the British Islands were shaded into six divisions, these 

 shadings showing the districts where the mean annual rainfall 



1st does not amount to 25 inches 

 2nd is from 25 to 30 ,, 

 3rf .. 30 .. 40 .) 



4th ,, 40 ,, 60 ,, 



5th ,, 60 ,, 80 ,, and 



6th above 80 inches. 



On the map exhibited on the wall these divisions are shown 

 by three tints of blue and three of red, — the blue showing a 

 rainfall exceeding forty inches annually, and the deepest tinted 

 blue the regions of largest rainfall ; and the red a rainfall less 

 than 40 inches, th; lightest tint marking off those parts of 

 England where the rainfall is least, or where it is less than 25 

 inches annually. 



The regions of heaviest rainfall, marked off by 80 inches 

 annually, or upwards, are these four : — 



(1) The greater part of Skye, and a large portion of the 

 mainland to the south-east, as far as Luss. 



(2) The greater part of the Lake District. 



(3) A longish strip including the more mountainous portion of 

 North Wales, and 



(4) The mountainous district of the south-east of Wales. 

 The rainfall is also heavy on Dartmoor, and certain portions 



of the west of Ireland ; but in these parts it does not appear 

 quite to reach So inches. 



The West Highlands present the most extensive region of 

 heaviest rainfall in the British Islands. The mountain-masses 

 along whose slopes and plateaus the rainfall is precipitated, 

 offer a practically unbroken face of Highlands directly in the 

 course of the rain-bringing winds from the Atlantic. Particular 

 attention is drawn to the circumstance that these mountain- 

 masses present many lochs and valleys directly in the course of 

 these winds, up which therefore the winds are borne, and these 

 cooling as they ascend pour "down the deluges of rain which 

 deeply trench the sides of the mountains in the lines of their 

 water-courses. 



This region of heaviest rainfall lies so far to the north of 

 Ireland that the rainfall is not lessened by a previous partial 

 drying of the Atlantic winds in their passage thither. To 

 southward, however, it is quite different. Over the whole of 

 the extensive tract of Great Britain from Luss to the Lake 

 District there is not a single rain-gauge whose annual average 

 reaches 80 inches, even although a number of rain-gauges have 

 been planted in the higher districts, and in positions likely to 

 furnish approximately the maximum rainfall of these districts. 

 The diminished rainfall is no doubt due to the partial drying of 

 the Atlantic winds in their passage across Ireland before they 

 reach Southern Scotland. 



St. George's Channel and the Irish Sea open a free passage 

 to the south-westerly winds, here diverted into a more southerly 

 course, to the north of England and to Wales, and accordingly 

 where the mountain masses of the Lake District and of North 

 and South Wales oppose their course the rainfall over large 

 portions of these high districts exceeds 80 inches. 



The maximum falls in these four districts respectively are 

 185 '96 inches at The Styx, in the Lake District ; 128 '50 inches 

 at Glencroe, Argyllshire; Ii6'90 inches at Beddgelert, North 

 Wales; and gS'iS inches at Ty-Draw-Treherbert, South 

 Wales. 



The largest region of 60 to 80 inches rainfall is in the West 

 Highlands, surrounding the region of still larger rainfall of 80 

 inches and upwards, and it extends from the Crinan Canal to 

 beyond Loch Assynt in Sutherland. Then follow the hills to 

 the north of Galloway, the hills to the north and east of Dum- 

 friesshire, large portions of the Lake District, of North and 

 South Wales, of West Galway, the mountainous districts of 

 Kerry, and Dartmoor in Devonshire. 



