Page:Popular Science Monthly Volume 16.djvu/202

 is, the mean of all the hourly observations of each of these quantities while in the square—is found and entered. Beside each is placed its daily range, by which is to be understood the mean of the différences between the daily maxima and minima for the number of days the ship was in the square.

Throughout a wide zone on both sides of the equator, the barometer has a remarkably regular oscillation, attaining two maxima and two minima every day. In order to discover the extent of this phenomenon, the hours of its recurrence, and the amplitude of the alternate rise and fall, the daily record of the barometer is carefully examined, and, whenever the phenomenon is found clearly defined, the hours of the two extreme lowest and two extreme highest readings, together with the readings themselves, are noted and tabulated on the blank. To save repetition, I will state here that all compilations of the barometer are reduced to 32° Fahr. and sea-level. As two more items of interest, the highest and the lowest readings of the barometer and dry-bulb that occurred during the whole time the ship was in the square are noted and entered. A miniature chart of a five-degree square, but on a sufficiently large scale to allow of considerable accuracy, is printed on the Form, for plotting the ship's track: the position of the ship being determined at 8, noon, and 8 of each day, these points afford the data for tracing the track. Whenever currents exist, their set in points and their velocity in knots and tenths of a knot per hour are written on this little chart at each noon position; an arrow is also projected from the track to indicate their direction. At every noon the temperature of the sea-water at the surface is very carefully taken, and entered on the chart on Form B, beside the noon position of the ship; it assists in coming to a conclusion as to the existence of currents.

This completes the tabulated portion of the compilation on Form B; but, to unite the whole, to trace the connection of the several related quantities, and to describe such matters as could not be otherwise noticed, ample remarks are made on the blank.

Thus, at a glance, are all the observations of a ship in each square visible on a single sheet; it is not assumed that what she experienced prevailed over all the expanse of the square; her track lies before us on the little chart that represents that square, and the tabulated observations beside it relate to that track only—a single line! If we have the observations and tracks of a large number of vessels, all over every part of a five-degree square for each month—in other words, if we have a multitude of sheets of Form B compiled—it is evident that we can easily deduce the meteorological features of that square, and judge whether they be like or dissimilar throughout its entire extent.

To follow up the fate of Form B, compilations having been made on many thousand sheets of it from all the log-books and journals that