192 THE POPULAR SCIENCE ' MONTHLY. 



The method of compilation having been described, the charts will 

 now be easily understood. 



The figure on page 191 is a fac-simile of two adjoining squares of a 

 monthly sheet — February. They cover the small area of ocean west 

 of the Mexican coast between the parallels of 15° and 20° north and 

 the meridians of 95° and 105° west. The month and square constitute 

 jointly the unit for which all the information is given ; for some 

 squares this is meager, for others full — always dependent on the length 

 of time ships were in the square and the number of log-books examined 

 for it. 



The explanation of one of the above squares — No. 106 — will afford 

 a key to the whole series. The figures inclosed by circles relate to 

 the winds ; those between the outer and the middle circle indicate its 

 duration and force; and those between the middle and the inner circle, 

 its percentage, from every alternate point. The points are inclosed 

 by parts of radii that extend from the outer to the inner circle : thus, 

 the two radii opening toward the upper left-hand corner, and containing 



the figures l^^-y inclose the N. W. point ; the two containing 



inclose the N. N. W. point, and so on to the right through North, N. 

 N. E., etc. If we add together the hair-line figures between the outer 

 and the middle circle, as 226, 85, 76, etc., they will amount to 1,062 ; 

 add to this the figures under the heading " calm " (38), and " var." (wind) 

 (10), and the total is 1,110. This means that if the hours all the vessels 

 spent in the square were added together, the total would be 1,110 

 hours, or 46 days 6 hours ; this is the whole period of observation in 

 this square for this month ; it is composed of fragments of February 

 collected from many years. An hour is the unit of observation, and a 

 vessel had to be a whole hour in a square in order to have it constitute 

 an observation. In this square, then, there were 1,110 observations of 

 the wind : of this number, it was 226 hours from the N. W. ; 85 hours 

 from the N. N. W. ; 76 hours from North, etc. The small dark figures 

 to the right of the hair-line figures indicate the force of the wind : 

 thus, the 3 annexed to 226 signifies that the mean force for those 226 

 hours was 3, according to the scale already given. The number of 

 hours of wind from any one point may have been the experience of 

 one or several vessels ; and that number may be composed of hours of 

 light, gentle, fresh, and strong breezes ; but, however such variations 

 may have occurred, the mean force is indicated by the small dark 

 figures. 



It would, no doubt, be desirable to give the percentage of different 

 forces of the wind from each point, but this would greatly detract 

 from the simplicity and ease of consultation of the charts. The defect 

 is, however, approximately met by giving, as is done, the extreme 

 variations of the wind, i. e., the number of calms as well as of heavy 



