EEPOKT OF THE SECRETARY. 21 



defraying the cost of making the reductions and numerical computa- 

 tions.* This work may be considered an extension of the Winds of the 

 Northern Hemisphere, prepared by Professor Coffin, and published by 

 the Institution in 1853. It is designed to show principally — 



First. The mean direction in which the lower currents of the atmo- 

 sphere move over all parts of the surface of the earth. 



Second. The ratio that the progressive motion bears to the total dis- 

 tance traveled. 



Third. The modification that the currents undergo in different seasons 

 of the year. 



Fourth. The direction in which the forces act that produce these mod- 

 ifications. 



Fifth. The amount of intensity of these forces as reckoned on the 

 same scale as that which determines their mean annual direction. 



The data used for elucidating these points consist of series of obser- 

 vations of winds made at 3,223 different stations on land and during 

 numerous voyages at sea, extending from the parallel of 82° 16' north 

 to beyond the parallel of 75° south latitude, altogether embracing an 

 aggregate period of over eighteen thousand five hundred years. The 

 whole material is arranged in the form of tabular series, and for conve- 

 nience in discussion the entire surface of the earth is divided into thirty- 

 six zones by parallels of latitude drawn five degrees asunder, com- 

 mencing at the North Pole and proceeding southwardly. The method 

 of reduction is not that which has usually been adopted of obtaining 

 the prevailing direction or the point from which the wind blows most 

 frequently, but the traverse of the whole is made out in the same man- 

 ner as that of a ship at sea. Suppose a particle of air to start at a given 

 point and to move with uniform velocity for nine days, viz, from the 

 northeast for a period of three days, southeast four days, south two 

 days, at the end of this time the particle will be found at a certain point; 

 the distance from the starting-point and the direction of the line joining 

 the ending-points will be formed by a traverse. In this way the result- 

 ant direction of the wind is determined for a given place. If no deflect- 

 ing force was exerted, the mean direction and relative progress of the 

 wind would be the same for each month of the year; but as this is not 

 the case, by comparing different resultants, an idea of the deflecting 

 force is obtained, or, in other words, of the monsoon influence which tends 

 in different seasons of the year to vary the resultant direction. 



At the time of the death of Professor Coffin in 1873, the principal ser- 

 ies of tables was mainly completed, yet there were blanks left to be filled. 

 The supply of these deficiencies and the preparation of the illustrative 

 plates was undertaken without pecuniary compensation by his son and 

 successor in Lafayette College, Prof. Selden J. Coffin, who executed the 

 work with a feeling of pious regard for the memory of a venerated parent, 

 interest in science, and a devotion which merits special commendation. 



*Altogether 80 persons were engaged in these reductions, the principal assistant of 

 Professor Coffin being Mr. Henry Mansfield, a skilful computer. 



