XVIU EXPLANATION OF THE PLATES. 



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 pa- 

 rallels 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 slough- 

 ing off from the lower current, and a joining and a turning back with the upper 

 current. This phenomenon is represented by tlie small featherless and curved 

 arrows in the periphery on the polar side of the calm belts of Cancer and Ca- 

 pricorn. 



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 tlie atmosphere is resorted to in order to show the unequal dis- 

 tribution 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). 



PiiATE IV, (§ 723) is intended to illustrate the extreme movements of the 

 isotherms 50°, 60°, 70°, etc., in the Atlantic Ocean during the year. The con- 

 nection between the law of this motion and the climates of the sea is exceedingly- 

 interesting. 



PiiATE 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. 



