340 On the Distribution of Temperature, etc. 



Mediterranean agreed sensibly with the mean winter tem- 

 perature of the air at its surface. The later view, which 

 ascribed the temperature of the deep water in the Mediter- 

 ranean to the Atlantic water flowing over the ridge at Tarifa, 

 which has the same temperature as the Mediterranean water 

 within the ridge, left out of account the fact that there is on 

 the whole an outflow of pure Mediterranean water at the 

 bottom which affects the temperature and density of the 

 Atlantic water outside. In sea water, owing to its saltness, 

 convection currents are set up more actively by cooling than 

 is found to be the case in fresh water. In lakes, however, and 

 especially in those situated in mountainous districts, the pro- 

 duction of convection currents is powerfully assisted by local 

 differences of climate. These are due to differences of exposure, 

 to radiation, and to prevailing winds. Such local differences 

 of climate produce local differences of temperature, and con- 

 sequently of density in the superficial layers. If we compare, 

 for instance, the observations made at Ardlui and at Inversnaid 

 on I5th October, we find that the mean temperature of the 

 first 20 fathoms is 48-24 at Inversnaid, and only 46-58 F. at 

 Ardlui. The stations are only three miles apart, and yet there 

 is a difference of 1-66 F. in the mean temperature of the first 

 20 fathoms. Another cause affects the distribution of tem- 

 perature in a lake, namely, the drainage, but this more particu- 

 larly affects shallow lakes or basins. It is probable that the 

 most powerful means of supply and removal of heat is direct 

 radiation. The most powerful mixing agency is the wind. 



