688 OCEANOGRAPHY 



nounced than in the ocean. The relations of oceanography to many 

 other sciences can best be exemplified by a consideration of the dis- 

 tribution of temperature in the waters of the ocean. Nearly all the 

 sun's rays falling on water are at once diffused downwards to at least 

 600 feet. So great is the thermal capacity of water that a unit of heat 

 only raises the temperature one degree, while the same amount will 

 raise the temperature of rocks four or five degrees. 



It is well known that our planet is surrounded by three atmo- 

 spheres : one of oxygen, one of nitrogen, and one of water-vapor. In 

 the case of oxygen and nitrogen a complete mixture takes place 

 throughout the whole atmospheric envelope. A complete mixture 

 never takes place in the case of water-vapor, because its equilibrium 

 is continuously disturbed by changes of temperature, which may 

 reduce the vapor to the liquid or solid state; evaporation and con- 

 densation, freezing and melting, are ceaseless at the surface of the 

 earth. It has been shown by numerous observations that in the open 

 ocean far from land the daily fluctuations of temperature in the 

 surface waters do not exceed one degree F. Hence the atmo- 

 sphere over the ocean may be regarded as resting on a surface 

 the temperature of which is practically uniform at all hours of the 

 day. This is in striking contrast to what takes place on the land 

 surfaces, where solar and terrestrial radiation produce a very wide 

 daily range of temperature. On the sand of the Sahara and the 

 American deserts the temperature ranges about 100 from three A. M. 

 to three p. M. The temperature of the air immediately over the ocean 

 has a slightly greater daily range than that of the water, - - being 

 some three or four degrees F., -- but this is in no way comparable 

 to the enormous daily range of the air resting on the land surfaces. 

 Here we come on one of the prime factors of meteorology, which 

 must be considered in connection with some other facts. As the 

 diurnal oscillations of the barometer occur alike over the sea and 

 land, it follows that this diurnal oscillation is caused by the direct 

 and immediate heating of the molecules of the air and its aqueous 

 vapor by solar radiation. Air with a large quantity of water-vapor 

 absorbs more of the sun's rays, becomes in consequence more heated, 

 and is specifically lighter than dry air; hence air ascends in cyclonic 

 and descends in anti-cyclonic areas. The diurnal variation in the 

 elastic force of the vapor in the air is seen in its simplest form over 

 the open ocean, and the diurnal variation in the force of the wind, 

 and the diurnal variation in the amount of cloud are both much less 

 over the open ocean than over the land. All these conclusions 

 derived from observations at sea go a long way towards a rational 

 interpretation of many atmospheric phenomena, such as the un- 

 equal distribution of the mass of the earth's atmosphere, the as- 

 cending currents in cyclonic areas, the descending currents in anti- 



