22 REPORT OF THE SECRETARY. 



The Iiistitutiou also availed itself of the meteorological knowledge 

 and skill in investigation of Dr. Alex. Woeikof, of the Imperial Geo- 

 graphical Society of Russia, during his late visit to this country, for a 

 series of deductions from the tables and charts which the untimely 

 death of Professor Coffin prevented him from undertaking. 



Perhaps one of the most important additions to the meteorology of 

 the present day is the establishment of the dependence of the force and 

 direction of the wind upon the pressure of the atmosphere at different 

 points. This subject was especially studied by Professor Espy and 

 Professor Coffin, in this country, and by Professor Buys Ballot, in Hol- 

 land, and later by Professor Buchan, in Edinburgh. This relation may 

 be expressed as follows : At the surface of the earth the wind blows from 

 a region of high pressure to a rcgioii of low pressure, and is deflected to the 

 right oiving to the rotation of the earth ; while in the region above a reverse 

 icind, deflected to the left, is talcing place. Professor Buchan has rendered 

 great service to meteorology by extending this law to the general phe- 

 nomena of the winds of the globe. For example, during the winter 

 months the land becomes cold by radiation below the temperature of 

 the sea, and, consequently, the air is more condensed over the former 

 than over the latter, the barometer stands higher, and currents of air 

 in accordance with the law in question tend to pass at the surface of 

 the earth from the land to the ocean, and in an opposite direction 

 above, while the reverse phenomena take place in summer. 



It is fully established that there are on the surface of the earth five 

 systems of winds, which roughly correspond with the zones of climate 

 and temperature, and that the boundaries of these systems vary in lati- 

 tude with the change in declination of the sun. In the torrid zone the 

 resultant of the wind is from an easterly direction toward a variable 

 middle line, giving rise to Avhat are called the trade-winds. In the tem- 

 perate zones the average direction of the wind is from the west ; and 

 again, in the arctic and antarctic regions, the resultant is from an east- 

 erly direction ; and, furthermore, the limits of these systems of winds 

 are connected with regions of high or low barometer. Thus, in the equa- 

 torial regions, the barometer above the middle line is below the average 

 height of 30 inches, while along the northern and southern limit of this 

 region there is a belt of high barometer, and again on the northern and 

 southern limit of the winds of the temperate zone there is a belt of low 

 barometer. The direction of the wind in these several regions and the 

 belt of high and low barometer are rei'crred to the unequal action of the 

 heat of the sun in rarefying the air at the equator, causing an indrawing 

 current at the surface of the earth, which takes a westerly direction on 

 account of the revolution of the earth on its axis, and a current toward 

 each pole, which, from the same reason, has a direction from the west. 

 The equatorial current, cooling above, descends by its superior weight 

 at the northern limit of the trade-winds, producing the belt of high 

 barometer, from which, in opposite directions, two currents move, one 



