ON THE MONTHLY ISOTHERMAL LINES OF THE GLOBE. 87 
et la Zélée’. These three works, with Clerk’s ‘Daily abstract of meteorologi- 
cal observations made on board the Pagoda,’ and King and FitzRoy’s ‘ Nar- 
rative of the surveying voyages of the Adventure and Beagle,’ describing 
their examination of the southern shores of South America, have rendered 
it possible to deduce the isothermals of the Southern Hemisphere much 
more extensively than could have been done a short time ago, and thus to 
obtain an approximate determination of the temperature of the southern half 
of the globe. I have also made use of the following journals :—Vaillant’s 
‘Voyage autour du Monde sur la Bonite ;? Du Petit Thouars’ ‘ Voyage 
autour du Monde sur la Venus;’ Duperrey’s ‘ Voyage autour du Monde sur 
la Coquille;’ Freycinet’s ‘Voyage autour du Monde sur l’Uranie et la 
Physicienne’ (which affords particularly abundant data for the tropical re- 
gions); Liitke’s ‘ Voyage autour du Monde sur le Seniavine;’ Meyen’s 
‘Reise um die Erde;’ Rafaele de Cosa’s ‘Corsi di osservazioni meteoro- 
logiche fatte nella Zona torrida a bordo del Vesuvio ;’ Hasskarl’s ‘ Me- 
teorologische Waarnemigen op drie Reizen van en naar de Oostindien;’ a 
journal of Dieffenbach during a voyage from England to New Zealand, and 
one of Schaeyer’s on a voyage from England to Australia; Reynolds’s ‘ Voy- 
age of the Potomac during the circumnavigation of the Globe,’ and Erman’s 
‘ Observations on Board the Krotkoi’ in his Russischen Archiv. Lastly, I 
have used of older voyages, those of Peron and Baudin, La Perouse, Dentre- 
casteaux, Lisianski, Krusenstern, Chamisso, and the journals of Lawson, 
Peters and Newbold. 
_ Although, by reason of the smaller variation of the temperature on the 
surface of the Ocean, observations even of very short period give approxi- 
mate results, yet the mass of materials which one fancies at first sight one 
has at command contracts exceedingly in its dimensions on a nearer inspec- 
tion; for as on land, stations of observation are unnecessarily crowded in 
some places and altogether wanting in others, so also at sea, there are much- 
frequented routes, and on the other hand extensive tracts which are hardly 
ever traversed. The influence of season encounters the inquirer the more 
frequently in sea observations, because the prevailing winds of different parts 
of the year determine the most favourable season of navigation for particular 
routes. Against this inconvenience, we may place the advantage which sea 
observations possess of getting rid of the often very uncertain correction for 
the influence of elevation. 
_ From the above materials I haye constructed maps both on an equatorial 
and polar projection of the isothermal curves of January and July ; the lines 
in the January map being for every 4° of Reaumur (every 9° of Fahrenheit), 
and those in the July map for every 2° of Reaumur (every 44° of Fahren- 
heit). It is in these months that the inquiry can be best pursued into the 
higher latitudes, as in one of them, the southern pole, and in the other, the 
northern pole, are most nearly approached by navigators. These two months 
also represent the extreme difference of the distribution of temperature within 
the annual period, the other months forming intermediate steps between the 
two extremes. In addition to the isothermal lines, others are drawn which 
may be termed lines of normal temperature : however different the tempera- 
ture may be at different parts of the same parallel of latitude, yet every 
parallel of latitude has a determinate mean temperature, which may be found 
by a graphical interpolation, and which is the proper mean or normal tem- 
perature of the parallel at that particular season. Places where the tempe- 
rature agrees with this value have a normal temperature ; those where it is 
_ lower are relatively cold, and those where it is higher relatively warm. If we 
