65'2 PHILOSOPHICAL TRANSACTIONS. [aNNO I749. 



and cooling by the sun's presence or absence ; or by tides in the atmosphere, 

 occasioned by the moon's attraction, like the tides in the ocean ; though it is ac- 

 knowledged that the alleged effect is very far from uniformly following the regu- 

 lar pretended cause. 



Though Mr. H. be not satisfied therefore with those ideas on the cause of the 

 alteration in the weight and elasticity of the air, he contends at least that this 

 ^Iteration, not merely accompanies the turbid state of the atmosphere and the 

 descent of rain, but is indeed the very immediate cause of them ; though his 

 illustration of the manner in which this is effected be not satisfactory, by noticing 

 that exhausting the air out of a receiver by the air-pump, the inside becomes 

 turbid or misty ; since this phenomenon is otherwise accounted for. 



Mr. H. concludes with these reflections : we find many are solicitous to 

 foretell the weather from the rising or falling of the barometer, and endeavour to 

 form certain rules for that purpose : so that this seems to be the only thing which 

 persons, not otherwise versed in natural philosophy, may expect from this ma- 

 chine. But these things, which we would have so conjoined, do not seem to 

 be uniformly connected. For the barometer cannot properly, and of itself, show 

 any thing but the increase or decrease in the elastic force of the air ; whereas 

 the weather depends on various exhalations, existing at the same time, or to- 

 gether in the air, or not existing, or at least not present in the same quantity. 

 But it may easily be, that the elasticity of the air is diminished, yet from a de- 

 fect of a sufficient quantity of exhalations in it, little or no sensible change in the 

 weather may take place. The same happens on the contrary, by too great a 

 quantity of exhalations in the air, though tlie rising of the quicksilver may most 

 evidently show that the elastic force of the air is increased. Hence then as these 

 things coincide as it were by some accident, no certain prediction can be made 

 of a change of weather, either by the rising or falling of the barometer. But 

 we may safely enough make a kind of negative conclusion from it : for constant 

 agreement of observations shows, that when foul weather has followed a descent 

 of the mercury, then fair will not take place till the mercury begins to rise again; 

 or if the ascent of the mercury has been attended with fair weather, then there 

 will be clouds or rain till the mercury has begun to fall. Yet the descent of the 

 mercury does more frequently predict foul than fair weather ; and its rise more 

 often fair than foul ; as is shown by very manifold experience. 



On the Effects oj' Lightning in Destroying the Polarity of a Mariner s Compass. 

 By Copt. John fVaddell. N° 492, p. HI. 



On Jan. 9, 1 748-9, the new ship Dover, bound from New York to London, 

 being then in lat. 47° 30' north, and longitude 11° 15' west, from London, met 

 with a very hard storm of wind, attended with thunder and lightning, as usual, most 



