554 



mirage is so common under the tropics, where the rays reach 

 us gliding over the surface of the ocean. (Brandes, in Gil- 

 bert's AnnaL, torn, xvii, p. 175.) 



In the Sanscrit, the phenomenon of the mirage is called 

 mriga trichna, " thirst or desire of the antelope," no doubt 

 because this animal, mriga, compelled by thirst, trichna, ap- 

 proaches those barren plains, where, from the effect of the 

 inflexion of the rays, he thinks he perceives the undulating 

 surface of the waters. 



NOTE E. 



The mean temperatures of the year indicate the tempera- 

 tures that would exist in the various places of the globe, if 

 the unequal quantities of heat emitted in different seasons, 

 and at different hours of the day and night, were uniformly 

 distributed in the space of a year. Since the latest inves- 

 tigations of the heat of the interior of the Earth, at various 

 latitudes and heights, we cannot consider as identical the 

 mean temperatures of the lowest strata of the atmosphere 

 and those of the stony crust of the globe. It has been often 

 said, that the mean temperatures were sufficient to deter- 

 mine by one single member the climates of different latitudes 5 

 but this assertion is not perfectly accurate. To determine 

 the climate, we must know the distribution of heat in differ- 

 ent seasons of the year : and two places, as Milan, and Pe- 

 kin for example, the mean temperatures of which (13°) are 

 the same, may have, the first a winter of -f 2*4°, and a sum- 

 mer of 22'8°, and the second a winter of — 3°, and a summer 

 of 28°. It is true, that, wherever the mean temperature of 

 the year is above 15°, we do not find a winter the mean heat 

 of which is below 0°. Joining by a curve (an isothermal) 

 the places, the mean annual temperatures of which are the 



