THE WEATHER AND THE SUN. 483 



have noticed, that the varying warmth on which the processes of vege- 

 tation depend, corresponds with the varying diurnal course of the sun. 

 So soon as this was noticed, and so soon as the periodic nature of the 

 sun's varying motions had been ascertained, men had acquired in effect 

 the power of predicting that, at particular times or seasons, the weather 

 on the whole would be warmer than at other seasons. In other words, 

 so soon as men had recognized the period we call the year, they could 

 predict that one half of each year would be warmer than the other 

 half. Simple as this fact may seem, it is important to notice it as the 

 beginning of weather-prediction ; for, as will presently appear, it has 

 an important bearing on the more complex questions at present in- 

 volved in the prognostication of weather-changes. 



It became manifest, almost as soon as this discovery had been made, 

 that the weather of particular days, or even of weeks and longer pe- 

 riods, could not, by its means, be predicted. A week in summer may- 

 be cold, and a week in winter may be warm; nor, so far as is even yet 

 known, is there a single part of any year the temperature of which 

 can be certainly depended upon, at least within the temperate zone. 

 In certain tropical regions there are tolerably constant weather varia- 

 tions ; but, so far is this from being the case in the temperate zones of 

 either hemisphere, that it is impossible to affirm certainly, even that 

 during a week or fortnight at any given summer season there will be 

 one hot day, or that during a corresponding period in winter there will 

 be one day of cold weather. 



It became manifest also, at an early epoch, that terrestrial condi- 

 tions must be intimately involved in all questions of weather, since 

 the year in different countries in the same latitudes presents different 

 features. Such differences are of two kinds those which have a ten- 

 dency to be constant, and those which are in their nature variable. 

 For example, the annual weather, in Canadian regions having the same 

 range of latitude as Great Britain, differs always to a very marked 

 degree, though not always to the same degree, from that which pre- 

 vails in this country ; here, then, we have a case of a constant differ- 

 ence due unquestionably to terrestrial relations. Again, when we have 

 a hot or dry summer in this country, warm or damp weather may pre- 

 vail in other countries in the same latitudes, and vice versa ; differences 

 of this kind are ordinarily 1 variable, and in the present position of 

 weather-science are regarded as accidental. 



1 1 use this qualifying word, because some differences of the kind are more or less 

 regular. Thus, when there is a dry summer in certain regions in the west of Europe, 

 there is commonly a wet summer in easterly regions in the same latitude, and vice versa, 

 the difference simply depending on the height at which the clouds travel which are 

 brought by the southwesterly counter-trade winds. When these clouds travel high, they 

 do not give up their moisture until they have travelled far inland or toward the east ; 

 when they travel low, their moisture is condensed so soon as they reach the western 

 land-slopes. It is not uncommonly the case again that, when we in England have dry 

 summers, much rain falls on the Atlantic, and our drought is simply due to the fall of 



