Hydrography^ and the Art of Navigation. 395 



requesting, that, during the whole voyage (if the Botiite, note 

 sliould be taken, both by day and night, andjrom hour to hour, 

 of the temperature of the air, of the temperature of the surface 

 of the sea, and of the atmospheric pressure. They will like- 

 wise authorize us to hope that these observations will continue 

 to be made with the same zeal, of which an example has beea 

 given by the officers of the Uranie, the Coquille, the Astrolabe, 

 the Chevrettff and the Loiret, At the same time, if unforeseen 

 circumstances require the omission of part of this labour, j| 

 would be desirable that the sacrifice should first be made of 

 what is least essential. The details upon which we are about to 

 enter, seem to us calculated, in such cases, to guide the selection 

 to be made by the commander of the expedition. 



Observations designed to characteii^e the present state of the 

 Ghbe in regard to Temperature. — Has the earth arrived at a 

 permanent state with respect to temperature.? The solution of 

 this important question seems to require only the direct compa- 

 rison of the mean temperatures of the same place, taken at two 

 distant periods. But when we take into account the effects pro- 

 duced by local circumstances, when we consider to what an ex- 

 tent the neighbourhood of a lake, of a forest, of a naked or 

 wooded mountain, of a sandy plain, or one formed of meadows, 

 may modify the temperature, every one will perceive that such 

 thermometrical data alone will not be sufficient ; that it is neces- 

 sary, besides, to ascertain that between the periods in question 

 the country, and even the districts adjoining it, have undergone 

 no important change in their physical aspect and in the nature 

 of their cultivation. It is thus seen that the question becomes 

 singularly complicated, and although numerals are adduced, with 

 sufficient precision to admit of a definite estimate, they become 

 mingled with vague suspicions, which continually throw a scru- 

 pulous mind into a state of suspense. v/.-iiJiu vm 



Is there, then, no means of solving the difficulty ? These nre^U|8 

 exist, and are by no means of a complicated nature, for we have 

 only to observe the temperature in the open sea, at a great d^- 

 tame from continents. If, for this purpose, we make choice 

 of the equinoctial regions, it is not necessary that the observa- 

 tions should be continued for a series of years ; the maxima 

 temperatures observed in crossing the line on two or three occa- 



