208 Miscellaneous Intelligence. 



The effects due to air passing from the north southwards should 

 be greater, because, arriving at warmer regions, it would become 

 heated, and tend to dilate ; the contrary effect would occur with air 

 passing from the south to the north. But these effects would be 

 smaller, if the changes of tension which act as causes of the wind, 

 occurred to the south instead of in the north, because the air, driven 

 forward by expansion, would contract as it travelled northwards, 

 and vice versa. 



The variations of the barometer between the tropics is scarcely any 

 thing ; the winds are very regular, the cause is permanent, and the 

 equilibrium appears constant. From this regularity, it may be con- 

 cluded, that the north and south winds which we have, originate in 

 the north; for how can it be imagined that a south wind should ge- 

 nerally come to us from a place where the contrary wind is constant? 

 On this view, the south wind would generally be caused by a con- 

 densation, and the north wind by an expansion of the polar atmos- 

 phere, and the barometer ought to fall with south winds and rise with 

 north winds. This effect is usually observed. 



Though the south wind, in passing northward, is cooled, and de- 

 posits part of its vapour in clouds and rain, it would appear, from the 

 above reasoning, that such deposition is not the cause of the descent 

 of the barometer. If vapour were the cause, the effect ought 

 to occur in every place where there is rain ; but this does not hap- 

 pen. It may easily be conceived, that although the south wind may 

 ordinarily be caused by condensation of air in the north, and be ac- 

 companied by depression of the barometer, it may sometimes be due 

 to dilatation in the south, and then the barometer would rise *. 



14. NITROUS ATMOSPHERE OF TIRHOOT. 



Tirhoot is one of the principal districts in India for the manufacture 

 of saltpetre : the soil is every where abundantly impregnated with 

 this substance, and it floats in the atmosphere in such quantities, that 

 during the rains and cold weather it is attracted from thence by the 

 lime on the damp walls of houses, and fixes there in shape of long 

 downy crystals of exceeding delicacy. From damp spots it may 

 be brushed off every two or three days almost in basketsful. In con- 

 sequence of all this, the ground, even in hot weather, is, so damp that 

 it is extremely difficult either to get earth of sufficient tenacity to 

 make bricks, (the country being quite destitute of stones,) or, when 

 made, to find a spot sufficiently solid to sustain the weight of a house. 

 Even with the greatest care the ground at last yields, and the salt- 

 petre corrodes the best of the bricks to such a degree, that the whole 

 house gradually sinks several inches below its original level. Houses 

 built of inferior materials of course suffer much more ; one, of which 

 the inner foundations were of unburnt bricks, absolutely fell down 

 whilst I was at Mullye, and the family in it escaped almost by 

 miracle. My own house, which was not much better, sank so much, 



* Aimalqs de Chimie, xlv. 421. 



