1 90 Listructionsfor the. Scientific Expedition 



standards, whenever the ships are in company, will become checks 

 on each other through the medium of the register. Nor should the 

 opportunity be lost of comparing these standard barometers (by the 

 intervention of portable ones) with the standard barometer of the 

 Cape Observatory, and with that used at Port Arthur, Van Diemen's 

 Land, in the meteorological register kept by Sir J. Franklin's orders 

 by Mr. Lempriere, as well as with the standard at the observatory 

 at Paramatta, and with any other instrument likely to be referred to 

 as a standard or employed in research elsewhere. 



The general fact that the barometer at the level of the sea does 

 not indicate a mean atmospheric pressure of equal amount in all 

 parts of the earth, — but, on the contrary, that the equatorial pressure 

 is uniformly less in its mean amount than that at and beyond the 

 tropics, — was first noticed by Von Humboldt, and has since been de- 

 monstrated by the assemblage of many observations made during 

 voyages and on land by Schouw, as well as by other observations, 

 an account of which will be found in the Reports of the Meteoro- 

 logical Committee of the South African Philosophical Society for 

 1836 and 1837. This inequality of mean pressure is a meteorolo- 

 gical phenomenon of the greatest and most universal influence, as it 

 is, in fact, no other than a direct measure of the moving force, by 

 which the great currents of the trade-winds are produced ; so that 

 the measure of its amount, and the laws of its geographical distri- 

 bution, lie at the root of the theory of these winds. The progress 

 of barometric depression on approaching the line, and re- ascension 

 in receding from it, will therefore be watched with interest propor- 

 tionate to its intrinsic importance during the voyage outwards and 

 homewards. 



But it may very well happen that phenomena purely local, of the 

 same nature, may exist, not as cause but as effect ; in other words, 

 that the regular currents once established may, in particular loca- 

 lities, determined by the configuration of continents and by the in- 

 fluence of oceanic currents, or other causes, form permanent eddies 

 or atmospheric ripples, so to speak, under which the mean pressure 

 may deviate materially from the general average. An instance of 

 permanent barometric depression of this kind, in the neighbourhood 

 of the sea of Ochotzk, is mentioned by Erman ; and a second seems 

 to be pointed out in the neighbourhood of Cape Horn, by some re- 

 marks stated to have originated with Captain Foster ; and it is not 

 impossible that something of the same kind, but of an inverse cha- 

 racter, may be found to obtain in that remarkable district of Siberia 

 mentioned by Erman, where during winter clouds are unknown and 

 snow never falls ; and it is somewhat curious to notice that the 

 localities in question are not far from antipodes to each other. 



In the outward and homeward passages of the Ex])edition across 

 the equator (especially should the ships be delayed by calms), op- 

 portunity will be presented of determining the amount of diurnal 

 barometric fluctuation, apart from the interfering influence of land 

 and sea breezes, or their equivalents far inland, which in all land 

 observations encumber and disturb this somewhat obscure pheno- 



