OCEAN METEOROLOGY. 183 



dead reckoning and observation, it forms a kind of check on those two 

 methods. 



I will now enumerate some corroborating circumstances that must 

 be considered in connection with the difference between the position 

 by observation and that by account, ere this difference — its set and 

 velocity — be tabulated as one of the permanent, ever-flowing currents 

 of the ocean : 



I. Temperatuke. — Of two contiguous bodies of water — one hot, 

 the other cold — the latter, being specifically heavier, will displace the 

 former, and hence 2i permanent current is established, 



II. Evaporation. — Since no salts are taken up in the vapor, a body 

 of salt water from which great evaporation takes place will be specifi- 

 cally heavier than an adjoining one that gives off less vapor, and so 

 a continuous flow from the dense to the light fluid will be main- 

 tained. 



III. "Winds. — In a gale, the waves roll one after another in huge 

 volumes toward the point to which the wind blows ; and the friction 

 of the wind upon the water produces a temporary surface set to lee- 

 ward. 



IV. Difference of Barometric Pressure. — In gales of wind, it 

 is common for the barometer to fall from, say, 30*20 to 29-70 — half an 

 inch — in less than a day, and while the ship is passing over a compara- 

 tively small extent of ocean. Take a very extreme case, merely for 

 illustration. Suppose two contiguous square miles of ocean, the barom- 

 eter standing 30*20 over one of them, and 29*70 over the other. This 

 difference of half an inch in the barometer is equivalent to a difference 

 of about one quarter of a pound pressure per square inch of surface, 

 or 36 pounds per square foot. Taking 6,086 feet as the side of a 

 square mile, it will contain 37,039,396 square feet ; each square foot 

 sustains a difference of j^ressure of 36 pounds, so that there are in all 

 1,333,418,256 pounds more pressure on the square mile over which the 

 barometer stands 30*20 than on the one over which it stands 29*70. It 

 is evident that, in order to attain an equality of level, a very decided 

 temporary set must take place from the former square mile toward 

 the latter. 



Instead of confining the case to the impossibly small area of two 

 square miles, let us suppose a gradual fall of the barometer from one 

 part of the ocean to the other — such a fall, in fact, over such an area 

 as often comes within the experience of every naval ofiicer — and it 

 stands to reason that waves of the ocean, like those of the air, only 

 smaller and more sluggish, are consequent upon every change of the 

 barometer. 



V. Rotation op the Earth. — From being at' rest, suppose the 

 earth to begin to revolve, as now, from west to east. On starting, the 

 water of the ocean would, owing to its inertia, recede from the western 

 shores of all the continents, and, as the earth continued to revolve, it 



