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violent motion, find the refinance it meets with, pro- 
duce heat in other bodies, when paffing thro’ them, 
provided they be fmall enough. A large quantity 
will pafs thro’ a large wire without producing any 
fenfible heat ; when the fame quantity paffing thro’ 
a very fmall one, being there confined to a narrower 
paffage, the particles crowding clofer together, and 
meeting with greater refiftance, will make it red hot, 
and even melt it. 
Hence lightning does not melt metal by a cold fu- 
fion, as we formerly fuppofed. But when it paffies 
thro’ the blade of a fword, if the quantity be not ve- 
ry great, it may heat the point fo as to melt it, while 
the broad eft and thickeft part may not be fenfibly 
warmer than before. 
And when trees or houfes are fet on fire by the 
dreadful quantity, which a cloud, or the earth fome- 
times difcharges, muft not the heat, by which the 
wood is firft kindled, be generated by the lightning’s 
violent motion thro’ the refilling combuftible mat- 
ter ? 
If lightning, by its rapid motion, produces heat 
in itfelf as well as in other bodies, (and that it does, 
I think, is evident from fome of the foregoing experi- 
ments made with the thermometer) then its fometimes 
fingeing the hair of animals killed by it may eafily be 
accounted for. And the reafon of its not always do- 
ing fo may, perhaps, be this : the quantity, tho’ fuf- 
ficient to kill a large animal, may, fometimes, not 
be great enough, or not have met with refiftance 
enough, to become by its motion burning hot. 
We find, that dwelling houfes, ftruck with lightn- 
ing, are feldom fet on fire by it ; but when it paffies 
thro 3 
4 ; 
