XXX 



EXPLANATION OF THE PLATES. 



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 slougliing oft' from the lower current, and a joining and a turning 

 back with the upper current. This phenomenon is represented by the small feather- 

 bss 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 at- 

 mosphere 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 

 iii:quality in the arrangement over them of the airy covering of our planet. Except- 

 ing a small portion of South America, the belt between the parallels of 40° and G5° 

 cr 70° south may be considered to consist entirely of sea. This immense area of 

 ^ atcr surface keeps the atmosphere continually saturated with vapor. The specific 

 gravity of common atmospheric air being taken as unity, that of aqueous vapor is 

 about 0.6 ; consequently the atmosphere is expelled thence by the stea7n, if, for the 

 sidce of explanation, we may so call the vapor Avhich is continually rising up from 

 this immense boiler. This vapor 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 either into 

 clouds or rain, and the latent heat that is s?t free in the pioccss assists still forther 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 toward 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 liigh 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 vapor 

 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 con- 

 struction of this diagram to be correct, we have facts for the assertion that in the 

 austral regions the quantity of air that this vapor permanently expels thence is from 

 one twelfth to one fifteenth of the whole quantity that belongs to corresponding lati- 

 tudes north — a curious, most interesting, and suggestive physical revelation. 



Plates II. and III. are drawings of Brooke's Deep-sea Sounding Ajiparatus for 

 bringing up specimens of the bottom (§ 573). 



^'latr IV. (§ 723) is intended to illustrate the extreme movements of the isotherms 

 50% G0°, 70°, etc., in the Atlantic Ocean during tlie year. The connection between 

 the law of this motion and the climates of the sea is exceedingly interesting. 



Platk V. (§ 781) is a section taken from one of the manuscript charts at the Ob- 

 servatory. It illustrates the method adopted there for co-ordinating for the Pilot 

 Charts the winds as reported in the abstract logs. For this puqiose the ocean is di- 

 vided into convenient sections, usually five degrees of latitude by five degrees of 

 longitude. These parallelograms are then subdivided into a system of engraved 



