Hydrography , and the Art of Navigation. 41 



noctial regions might be expected to prevail. In these latitudes^ 

 it would be curious to ascertain to what distance from the coast 

 this anomaly exists ; and at what longitude the trade-wind re- 

 sumes, so to speak, its laws. 



According to the most generally adopted explanation of the 

 trade-winds, there ought constantly to be found, between the 

 tropics, a higher wind, blowing in an opposite direction from tliat 

 at the surface of the earth. Numerous proofs have been already 

 collected of the existence of this counter-current. Careful ob- 

 servation of elevated clouds, particularly those named '^ pom- 

 meUs^ ought to afford indications of great value to meteorology. 



The periods, strength, and extent of the monsoons will form, 

 in short, a subject for study in which, notwithstanding a multi- 

 tude of important observations on the subject, much yet remains 

 to be gleaned. 



Phenomena of the Sea. 

 On the means of drawing up Sea-water from great depths^ 

 and discovering in what proportion the two chief constituent 

 principles of atmospheric air are contained in it.* — Chemists 

 have long since proved, that water becomes impregnated with 

 the gases which rest on its surface. This absorption is effected 

 by means of a true chemical affinity exerted on the different 

 gases, and when its effects on oxygen and azote, the two princi« 

 pal constituents of atmospheric air, are carefully examined, it is 

 found to be much stronger in regard to the first than the se- 

 cond. It hence follows, that the waters of seas and rivers, from 

 being always in contact with the atmosphere, become at length 

 impregnated with a mixture of gases, among which oxygen pre- 

 dominates. Indeed, the very accurate experiments of MM. 

 Humboldt and Gay Lussac have proved that rain-water, the 

 water of the Seine, and snow-water, contain a mixture of oxy- 

 gen and azote, which amounts from 29 to 32 parts of oxygen in 

 100 ; while the proportion of oxygen in atmospheric air is con- 

 stantly equal to 21 parts, and this in all seasons and climates. 

 MM. de Humboldt and Proven9al have besides determined the 

 entire volume of mingled gases contained in water near the sur- 

 face, and it was 55 of the volume of water. 



• This section of the instructions wa? communicated to M. Arago by M. 

 Biot. 



