Hydrography^ and the Art of Navigation. 27 



observations are usually made at unsuitable hours, and with in- 

 struments either inaccurate in themselves, or improperly placed. 

 It does not now appear difficult to deduce the mean temperature 

 of the day from observations made at any hour ; thus a meteo- 

 rological table, whatever may be the hours noted in it, will be 

 possessed of value, by the mere condition that the instruments 

 employed will admit of being compared with the standard baro- 

 meters and thermometers. 



We think it proper to recommend these comparisons to the 

 officers of the Bonite. Wherever they can be effected, local me- 

 teorological observations will be of value. A collection from the 

 newspapers of countries will often supply what would otherwise 

 be obtained with difficulty. 



Magnetism. 



Diurnal Variations of the Declination. — Of late years science 

 has been enriched with a considerable number of observations 

 on the diurnal variations of the magnetic needle ; but the greater 

 part of these observations have been made either in islands or 

 on the western sides of continents. Corresponding observations 

 made on the eastern sides would at present be very useful. 

 They would serve, in fact, to submit to an almost decisive proof 

 the greater part of the explanations of this mysterious pheno- 

 menon which have been promulgated. 



The route of the expedition does not allow us to suppose that 

 the Bonite can remain long at points situated between the terres- 

 trial equator and the magnetic equator, such as Fernambouc, 

 Payta, Cape Comorin, and the Pelew Islands. Had it been 

 otherwise, we should have particularly recommended the erec- 

 tion of M. Gambey's beautiful instrument, in a firm position, at 

 a distance from every ferruginous mass, and that the oscillations 

 of the needle should have been attended to with the most scru- 

 pulous care.* 



• At any rate we shall here present the problem, which observations inacie 

 at the points mentioned would serve to solve. In the northern hemispherey the 

 point of a horizontal magnetic needle, which turns towards the nwthy mores 

 from the east to the teest from Q\ o'clock in the morning tol ^ in the afternoon, 

 and from west to east from 1^ o'clock a. m. to the following morning. Our he- 

 misphere cannot be peculiar in this respect ; the same effect produced on the 

 north point here must be produced on the south point to the south of the 



