4*4^ PHILOSOPHICAL TRANSACTIONS. [aNNO l/SQ. 



a little snow having fallen in the night, it was by 1 1 the next morning, mostly 

 melted away on the surface of the earth, except in several places in Bushy-Park 

 where there were drains dug, and covered with earth, where the snow continued 

 to lie, whether those drains were full of water or dry ; as also where elm-pipes 

 lay under ground ; a plain proof that these drains intercepted the warmth of the 

 earth from ascending from greater depths below them ; for the snow lay where 

 the drain had more than 4 feet depth of earth over it. It continued also to lie 

 on thatch, tiles, and the tops of walls." 



That the air nearest the earth will be most warmed by its heat is obvious ; 

 and this has frequently been taken notice of in the morning, before day, by 

 means of thermometers at different distances from the ground, by Dr. Miles, 

 at Tooting in Surrey ; and is mentioned in p. 526, of the 48th volume of the 

 Philosophical Transactions. 



The aurora borealis, which happens at the time the needle is disturbed by the 

 heat of the earth, is supposed to be the electricity of the heated air above it ; 

 and this will appear chiefly in the northern regions, as the alteration in the heat 

 of the air in those parts will be greatest. This hypothesis will not seem impro- 

 bable, if it be considered that electricity is now known to be the cause of thun- 

 der and lightning ; that it has been extracted from the air at the time of an 

 aurora borealis: that the inhabitants of the northern countries observe the 

 aurora to be remarkably strong, when a sudden thaw happens after severe cold 

 weather ; and that the curious in these matters are now acquainted with a sub- 

 stance, that will, without friction, both emit and absorb the electrical fluid, 

 only by the increase or diminution of its heat : for if the tourmalin be placed on a 

 plane piece of heated glass, or metal, so that each side of it, by being perpen- 

 dicular to the surface of the heating body, may be equally heated ; it will, 

 while heating, have the electricity of one of its sides positive, and that of the 

 other negative ; this will likewise be the case when it is taken out of boiling 

 water, and suffered to cool ; but the side that was positive while it was heating, 

 will be negative while it is cooling, and the side that was negative, will be 

 positive. 



For the sake of those who may be desirous "^^ ™^f ^'"^^1 Variation for each 

 ,,.,.. ^ , Month m the Year 1759. 

 of exammmg the dmrnal variations of the January 7 s 



needle very minutely, Mr. C. annexed a com- ^^"^"i^"^^ ^ ^^ 



plete year's observations ; and deduced from April. .... . . ........ '. .... '. . . . 12 26 



the regular variations during that time, the ^^X* *3 ^ 



mean diurnal variation belonging to each ju\y II.'. .................... 13 14 



month: whence it appears that the diurnal August 12 19 



... /• T T 1 September 11 43 



variation increases iiom January to June, and October 10 36 



decreases from June to December. November 8 9 



December 56 



