velocity. In like manner, the counter-trades, as they approach the poles, are going from latitudes where the parallels are larger to latitudes where the parallels are smaller. In other words, they diminish, as they approach the poles, the area of their vertical section; consequently there is a crowding out—a sloughing off from the lower current, and a joining and a turning back with the upper current. This phenomenon is represented by the small featherless and curved arrows in the periphery on the polar side of the calm belts of Cancer and Capricorn.
This dotted or shaded periphery is intended to represent a profile view of the atmosphere as suggested by the readings of the barometer at sea. This method of delineating the atmosphere is resorted to in order to show the unequal distribution of the atmosphere, particularly on the polar side of lat. 40° S.; also the piling up over the calm belts, and the depression—barometrical—over the equatorial calms and cloud ring.
The engirdling seas of the extra-tropical south suggest at once the cause of this inequality in the arrangement over them of the airy covering of our planet. Excepting a small portion of South America, the belt between the parallels of 40° and 65° or 70° south may be considered to consist entirely of sea. This immense area of water surface keeps the atmosphere continually saturated with vapour. The specific gravity of common atmospheric air being taken as unity, that of aqueous vapour is about 0.6; consequently the atmosphere is expelled thence by the steam, if, for the sake of explanation, we may so call the vapour which is continually rising up from this immense boiler. This vapour displaces a certain portion of air, occupies its place, and, being one third lighter, also makes lighter the barometric column. Moreover, being lighter, it mounts up into the cloud region, where it is condensed into clouds or rain, and the latent heat that is set free in the process assists still farther to lessen the barometric column; for the heat thus liberated warms and expands the upper air, causing it to swell out above its proper level, and so flow back towards the equator with the upper current of these regions.
Thus, though the barometer stands so low as to show that there is less atmosphere over high southern latitudes than there is in corresponding latitudes north, yet, if it were visible and we could see it, we should discover, owing to the effect of this vapour and the liberation of its latent heat, and the resulting intumescence of the lighter air over the austral regions, the actual height of this invisible covering to be higher there than it is in the boreal regions.
Taking the mean height of the barometer for the northern hemisphere to be 30 inches, and taking the 100,000 barometric observations used as data for the construction of this diagram to be correct, we have facts for the assertion that in the austral regions the quantity of air that this vapour permanently expels thence is from one twelfth to one fifteenth of the whole quantity that belongs to corresponding latitudes north—a curious, most interesting, and suggestive physical revelation.
Plates II. and III. are drawings of Brooke's Deep-sea Sounding Apparatus for bringing up specimens of the bottom (§ 573).
Plate IV. (§ 723) is intended to illustrate the extreme movements of the isotherms 50°, 60°, 70°, etc., in the Atlantic Ocean during the year. The connection between the law of this motion and the climates of the sea is exceedingly interesting.
Plate V. (§ 781) is a section taken from one of the manuscript charts at the Observatory. It illustrates the method adopted there for co-ordinating for the Pilot Charts the winds as reported in the abstract logs. For this purpose the ocean is divided into convenient sections, usually five degrees of latitude by five degrees of longitude. These parallelograms are then subdivided into a system of engraved squares, the months of the year being the ordinates, and the points of the compass being the abscissa;. As the wind is reported by a vessel that passes through any part of the parallelogram, so is it assumed to have been at that time all over the parallelogram. From such investigations as this the Pilot Charts are constructed.
