16 



MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



to suggest the cause of tlicse movements. Plate XI affords an example of nearly all the different 

 storm tracks delineated ou the international maps of the United States Signal Service for a period 

 of more than four years. We perceive that north of the parallel of 30° storm tracks in all 

 longitudes almost invariably i)ursue an easterly course, but generally they show an inclination 

 towards the north of east ; while witbin the tropics storm tracks almost invariably tend westerly, 

 with au inclination towards the nortli of west. We also notice that none of the storm tracks 

 delineated on the chart reaches down to the equator. The lowest latitude of any center of low 

 pressure which has been distinctly traced is (i.io N., and there are eight cases of cyclonic storms 

 whose paths have been traced to points south of latitude 10° N. 



25. It is not, however, to be uiulerstood that at the equator the wind does not sometimes rise 

 to the force of a gale, but rather that a i-egular system of cyclonic winds, with a considerable 

 depression of the barometer, has never been known to prevail directly under the equator. Hard 

 gales and violent squalls of wind do however sometiuies occur directly under the equator. This is 

 shown by various logs quoted in I'iddingtou's Memoirs. The following is an exauiple from the 

 log-book of the Winifred, quoted in I'iddington's Eleventh Memoir, pages 30 to 40 : 



The following is from the log-book of the Pyzul Curreem lor the same period : 



These observations show that directly under the equator we may have winds of a dangerous 

 violence, accompanied by frequent (ihanges in direction, indicating somewhat imperfectlj' a cyclonic 

 character, and accompanied by sudden oscillations of the barometer, which are very unusual near 

 the equator. Within six degrees of the equator the depression of the barometer has, however, 

 never been found sufBcientlj' great, and the depression has not been maintained with suflicient 

 steadiness to enable us to identify an area of low pressure in its progress from day to day. 



26. Although violent gales do sometimes occitr directly under the equator, they are of very 

 rare occurrence. This is shown by Maury's Storm Chart of the North Atlantic Ocean, which gives 

 th(i number of gales which have been observed on the Atlantic Ocean in different latitudes from 

 the equator as far north as latitude (JOO. Ou this chart the ocean is divided into squares by parallels 

 of latitude drawn at intervals of five degrees from each other, and meridians of longitude at intervals 

 of five degrees. The following table presents a summary of the results of this chart. Eath S(]uare 

 of the table ('ontains three numbers. The, first shows the nuniber of observations within the given 

 S(inare, each observation representing a period of eight hours. The second shows the number of 

 gales reported, and the third shows the average number of gales occurring in a hundred observa- 

 tions. Thus in the square included between the parallels of 40° and 45° of north latitude, and 

 between the merulians of 45° and 50° west longitude from Greenwich, the first number is 1,863, 



