Page:Popular Science Monthly Volume 16.djvu/206

 The method of compilation having been described, the charts will now be easily understood.

The figure on page 191 is a facsimile 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 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 hairline 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