247 
are so just, and his satire so pointed, as to give me some appre- 
hension for the fate of my present letter, when subjected to his 
criticism and keen raillery. He reminds me of Dr. Halley’s con- 
jecture, that the world on which we dwell may have another 
habitable world within it, to which may be attached a system of 
subterranean luminaries, resembling those which yield light to our 
upper world, but necessarily moving within a smaller sphere*. He 
sarcastically asks, whether our volcanos are any thing but the grand 
outlets of the furnaces, and our veins of bitumen any thing more 
than the clogged-up chimnies of this inner world ? The dread of 
criticism must not, however, deter me — I have engaged myself to 
the work, and therefore must not shrink. 
Having laid before you the several opinions which have been 
published, respecting the formation of coal, and having found n^- 
self under the necessity of stating my suspicion of their insuffi- 
ciency, to account satisfactorily for the origin of this substance, it 
becomes a duty to offer any observations which may seem to be 
likely, in the least, to elucidate a subject involved in so much mys- 
tery. Bat be assured that, whilst doing this, I am not ignorant of 
the difficulties which surround me ; nor, knowing how considerable 
these are, do I presumptuously expect to establish an impregnable 
system. On the contrary, I fear, that many unforeseen and power- 
ful objections may arise, against opinions thus hazarded, on a 
question which has been left in a great measure undecided, after 
careful discussion, by men of eminent abilities. If, on the one 
hand, I experience the mortification of differing from opinions 
which have been generally received ; it is, on the other hand, highly 
satisfactory to know, that this difference is so little, as, in some 
instances, to consist merely in an extension, and in others, in little 
more than a modification, of those opinions. Thus the explana 
* Miscellanea Curiosa, p. 43. 
