160 Dr. wilson’s Anfwer to the 
by the known laws of vifion ; and hence arguments retting 
upon fuch principles may be denominated optical ones. On 
the other hand, when fpots are contemplated near the middle 
of the dllk, a great variety of changes are obferved in them, 
which depend not upon pofition, but upon certain phyficai 
caufes producing real alterations in their form and dimenfions. 
It is plain, that arguments derived from the conlideration of 
fuch changes, and which, on that account, may be called 
phyjical arguments , can aftift us but little in invettigating their 
third dimenfions; and, from the nature of the thing, mud 
be liable to great uncertainty. The author of the Memoire, 
in p. 51 1. &c. takes new ground, and proceeds with a num- 
ber of objections, depending upon that fort of reafoning which 
we have laid defined. 1 mud take notice, that a certain dittinc- 
tion has been here overlooked, which in my paper I endea- 
voured to point out. Prefuming upon our great ignorance of 
many things which doubtlefs affect deeply the conttitution of 
that wonderful body the fun, I offered, in part II. an account 
of the production changes and decay of the fpots, confi- 
dered as excavations, in the mott loofe and problematical man- 
ner, ftating every thing on this head in the form of queries. 
This account, crude and imperfeCt as it is, appeared to me 
much lefs incumbered with difficulties than any other, and of 
this fome ftriking examples are there fet before the reader. But 
I have exprefsly owned, that many circumffiances ttill remained 
unexplained ; and upon the whole marked out the theory, if 
fuch it may be called, as very imperfeCt. Nature unquettiona- 
bly abounds with numberlefs unthought-of energies, and 
modes of working moft curioufly and moft wifely adapted to 
all fituations in the material world : and in regard to that 
fyftem of economy which is eftablifhed in the fun, producing 
5 ' there 
