188 NINTH ANNUAL REPORT OP 



and that it does not rise in one vertical column, but that there is a con- 

 stant ascension and absorption going on over the whole extent agitated 

 by the breeze. By way of exempUfication, and to render my exposi- 

 tion more clear, I shall direct your attention to the action which takes 

 place lietween soft and salt water, at the mouths of large rivers. Cap- 

 tain King observed a current of salt water running up the mouth of the 

 Santa Cruz, beneath the fresh water. In this case we cannot for a 

 moment imagine that the salt water rises in a body at any particular 

 spot, and returns as salt water to the sea ; but onl}^ that the salt under 

 current is gradually absorbed into the fresh stream above, at every 

 point as far as the salt water extends. 



Now suppose the sea breeze has a depth of 2,000 feet, and extends 

 60 miles into the interior of the country; it will not rise in a vertical 

 column of 2,000 feel in width, but will be gradually absorbed, by rising 

 in small portions into the upper current along the whole distance tra- 

 versed. The greater depth of salt water at the mouth of the river 

 corresponds to the greater velocity of the air on reaching the land. 

 The spot where the breeze is not felt has its counterpart in that point 

 of the bed of the river where the salt water ceases to flow against the 

 stream. The deep upper current from the southwest, which overlies 

 the shallow sea breeze, performs the part of the large body of fresh 

 water of the river, to which the other plienomena are merely secondary. 

 The power which propels the salt water up the bed of the river is the 

 difference in the weight of the fresh and salt water. In the sea breeze, 

 the air over the surface of the land is lighter than that over the sea at 

 the same elevation; and it is this difference in the weight of the two 

 columns of air which, in this case, is the propelling power. If the ba- 

 rometer was sufficiently dehcate, the rate of motion of the breeze might 

 be calculated with considerable precision. 



The heat of the sun materially affects the force of the winds at the 

 earth's surface in all parts of the world. The still air of evening is well 

 known to be in striking contrast with the breeze of midday. During 

 the night the air cools more rapidly next the ground than at a greater 

 elevation. The ascending currents cease with the heat of the sun, and 

 friction soon induces a calm. 



At the meeting of the British Association for the Advancement of 

 Science the year before last, I took occasion, in illustrating the action 

 of one class of storms wdiich agitate the atmosphere of Europe, to 

 point out the fact that the sun, during the summer months, in North 

 Britain, had the effect to reverse the motion of a thin stratum of air at the 

 surface of the ground ; while, durivg the day., a north current constantly 

 flowed above and a south current flowed below, diirhig the night the latter 

 was reversed. I have tf)und similar phenomena to prevail in the United 

 States. For some days, in Charleston, last January, I observed that the 

 wind was south during the heat of the day, and north in the morning. I 

 cannot stop now to discuss the cause of this phenomenon, though it is ex- 

 ceedingly interesting in a scientific point of view. But there is an analo- 

 gous effect produced, upon a grand scale, east of the Rocky mountains, 

 over the United States, as well as the British Possessions, that requires 

 to be noticed in this place. It has been most satisfactorily mnde out 

 by Pr(jtessor Coffin, that southerl}^ winds, in North America, are much 



