396 TRANSACTIONS OF THE 



northerly winds reign, are manifold. Firstly, with a north and northeast 

 wind, there is generally a difference of from 9° to 12° Fahrenheit, between 

 the wet and dry-bulb thermometers. 



With the northwest, which crosses lower mountain chains, and may come 

 from the North Atlantic, the difference is generally from 5° to 8° or 9°. 



Secondly, the atmosphere is generally clear; the sky blue; the sun shines 

 warmly; the nights are comparatively cold, and the summits of mountains 

 above four thousand feet high, are generally free from clouds. 



These phenomena are easily explained on meteorological grounds. The 

 presence of moisture in the air, either as imperceptible vapor or as cloud, 

 gives a white appearance to the sky, and veils the earth from the sun's 

 rays. It thus becomes a kind of shield, a protection from the warmth of 

 the sun. 



When moisture scarcely exists, and the air is dry, as it is in the Medi- 

 terranean with a north wind, in Egypt, in the desert of Sahara with south 

 winds, indeed, in all dry regions, the sky'is always blue, the sun shines 

 with great power, and at night, owing to rapid radiation of the earth's heat 

 into space, the air becomes comparatively cold. 



Such is the climate of the north Mediterranean coast with northerly 

 winds. The sky is clear and blue, the sun shines like a globe of fire, 

 which it really is, and its rays reach the earth with great power. The 

 nights are there clear, the stars shine with a brightness unknown in the 

 north, and the temperature of the air is cold compared with what it is in 

 the daytime. * * * The English climate is partly explained by the 

 above facts. The atmosphere above the British Isles is always loaded with 

 aqueous vapor, which gives to the sky its usual whitish color. The aqueous 

 vapor of the atmosphere shields the earth from the action of the sun's rays 

 during the day, and prevents radiation during night. 



Hence the coolness of our summer, as compared with that of the same 

 continental latitudes, where this aqueous shield is wanting. In winter, when 

 the sun is low on the horizon and its rays are feeble, the cloud atmosphere, 

 by promoting radiation, keeps in the heat previously acquired, and con- 

 tributes, with the gulf stream, to render the British winter milder than 

 that of the drier continental regions in the same parallel of latitude. The 

 influence of these meteorological conditions on climate has been well 

 explained of late by Professor Tyndall in his lectures on heat. It is also 

 beautifully illustrated by the meteorological observations of Mr. Glaisher 

 during his aeronautic ascensions. Once above the aqueous vapor and the 

 clouds, which extend several thousand feet high in our climate, a dry 

 atmospheric region is reached, where the sky appears intensely blue. The 

 sun's rays here have so much power that they scorch and blister the face 

 and hands, although the thermometer may be much below the freezing 

 point. The Mediterranean climate, when the north winds blow, is like this 

 upper region of our own atmosphere. The air, containing but little moist- 

 ure when these north winds reign, as they do during the greater part of the 

 winter, the sky is blue, and the sun shines through it fiercely, even in mid- 

 winter. It thus warms all the objects with which it comes in contact, and 

 which are sheltered from the wind, that is the entire under cliff. 



The northwest wind, called the mistral in this part of the Mediterranean, 

 usually blows from the south of France as a cold, dry, cutting wind, which 

 is much dreaded. One of the great climatic advantages of Mentone is its 

 complete protection from this wind by the Turbia Mountain, which sepa- 

 rates it from Nice. 



When the mistral blows, the sky remains blue and the sun shines 

 warmly. Sometimes, however, the northwest wind blows no longer as a 



