284 Mr. W. Brown on the Oscillations of the Barometer. 



being more than usually stormy. Whatever difficulties do 

 arise, however, it must be borne in mind that the situation 

 of this island is peculiar, having in summer a temperature be- 

 low that of parts of Europe on the north, and in winter above 

 that of part of the continent on the south; hence a very con- 

 siderable complexity in the directions of the currents must 

 occasionally occur. 



The results given in the first portion of this paper are de- 

 duced from the simple fact of the descent of the upper current 

 of the atmosphere, and are altogether independent of the im- 

 mediate cause of its descent; that it must always be descend- 

 ing in some portion or other of its course, to supply the place 

 of the air flowing in the surface current towards the equator, 

 is very evident; but the indications of the barometer show that 

 the acting cause of its descent at any particular time is not — 

 always at least — a deficiency or rarefaction of the air, such as 

 would be occasioned by the flowing of the lower current, were 

 its effect uncompensated by the arrival of air from above, for 

 the south wind often sets in when the barometer is high. I 

 have alluded to this subject before (Phil. Mag. Oct. 1843, 

 p. 280), noticing the effect of the difference of the opposite 

 currents with respect to the quantity of aqueous vapour in 

 each ; but though there appears no reason to doubt that effect 

 being, in a greater or less degree, as there supposed, I stated 

 a difficulty which it is probable does not exist, that of the 

 descending air being warmer than the air previously in its 

 place — an opinion derived simply from the fact of the upper 

 current having, from its origin, generally the higher tempera- 

 ture. Now it may very often be observed, though not always, 

 that the temperature does actually become colder immediately 

 at the change of the wind from north to south, though it rises 

 again on the continuance of the wind ; for when once it has 

 found its way to the surface, then of course, whatever its tem- 

 perature subsequently, it will continue there until its force be 

 overcome, one portion making way for the next following. 

 The cases in which the temperature rises immediately on the 

 change of the wind may be those in which the change either 

 takes place, not from an immediate descent of the current, but 

 simply from its advance (§ 16) from southern localities, where 

 it has previously descended, or perhaps been blowing for 

 some time, and advanced by reason of an increase of force; or 

 by its recession (§ 15) from north*. 



* This is a distinction which must be carefully borne in mind during the 

 reading of these remarks, and during all consideration of this subject, in 

 order to guard us from drawing inferences in any one particular case of a 

 change of wind, when its nature in this respect is not known. 



