Page:Popular Science Monthly Volume 16.djvu/207

 squalls and gales. The figures between the middle and the inner circle denote the percentage of wind from every alternate point: thus, of the 1,110 hours of observation, the wind was 20 per cent, of the time from the N. W.; 8 per cent, from N. N. W.; 3 per cent, was calm; and one per cent, light variable winds. The little table below the number of the square is to be read thus: of the 1,110 hours, 30 were calm; 10 were characterized by light airs flying all round the compass; no fog; 9 hours were rainy; no heavy squalls; and no light squalls. In the upper left-hand corner is the mean of the barometer, 30·05—the mean of all the hourly observations taken in the square during the month; under it is the mean daily range, 0·09 inch—the mean of the differences between the daily maxima and minima for 46 days. In the upper right-hand corner, under D. B. (dry-bulb) is the mean temperature of the air (in the shade), 80° Fahr.; the 5° under it is the mean daily range, both means obtained as for the barometer. Similarly, in the lower right-hand corner, is the mean temperature of evaporation and its daily range; and, in the lower left-hand corner, the mean temperature of the sea-water at the surface and its mean daily range.

The remarks within the circles are drawn from the experience of all the vessels that passed through the square in all the months of the year. It is their aim to fill up the outline character of the square afforded by the figures. While these remarks run through all the squares that have sufficiently similar features to be described together, the figures are applicable only to the square and month in which they appear. Thus are the climatic features of each small area of ocean delineated.

It should be understood that these charts merely exhibit the experience of the past reduced to a scale of probabilities for the future: if, then, the conditions stated in any square be not exactly realized, the system should not hence be condemned.

Let any navigator consider the degree of dependence he will place on a trustworthy record of a passage he once made; the confidence with which he will refer to that record—to the experience of a single voyage!—for his guidance in traversing the same ground again: now, it is not the record of one passage alone, but of many—all conveniently arranged, with every discernible error and inaccuracy eliminated—that is given in any square of these charts.

As a fair way of regarding them, let us consider the data of the square 106: of the 1,110 observations of the wind, it was 226 hours, or 20 per cent, of the whole time, from the N. W.; hence, at any future time, there are twenty chances for a K W. wind of a force of 3, against eight for a K N. W. wind, force 2; against seven for a N. wind, force 3; against three for calms and one for very light airs.

The observations of the direction of the wind on Maury's pilot charts have been incorporated with those extracted from log-books and journals of recent date, and, although they outnumber the latter, still