Diftance, Magnitude, &c. of ibe Fixed Stars, &c. 57 
that any very great progrefs can be made in this fubject for 
many years, or perhaps fome ages to come; the above outlines, 
therefore, of the ufe that may be made of the obférvations 
upon ‘the double ftars, &c. provided the particles of light 
fhould be fubjeé& to the fame law of gravitation with other 
bodies, as in all probability they are, and provided alfo that 
fome of the ftars fhould be large enough fenfibly to diminith 
their velocity, will, I hope, be an inducement to thofe, who 
may have it in their power, to make thefe obfervations for the 
benefit of future generations at leaft, how little advantage 
foever we may expect from them ourfelves; and yet very pof- 
fibly fome obfervations of this fort, and fuch as may be made 
in afew years, may not only be fufficient to do fomething, even 
at prefent, but alfo to fhew, that much more may be done 
hereafter, when thefe obfervations fhall become more numerous, 
and have been continued for a longer period of years. 
pretty near the truth, any fatellire revolving -round fuch a ftar, provided ‘the ftar 
was not either of lefs fpecific brightnefs, or of greater denfity than the fun, 
muft, if it appeared at its greateft elongation, at the diftance of one fecond only 
from its principal, be between three and four hundred years in performing one 
revolution; and the time of the revolution of the very {mall ftar near « Lyre, if 
it is a fatellite to this latter, and its principal is of the fame {pecific brightnefs and 
denfity with the fun, could hardly be lefs than eight hundred years, though 37’ 
the diftance at which it is placed from it, according to Mr. Herscuer’s obfer- 
vations, fhould happen to be its greateft diftance. Thefe periodical times, 
however, are computed from the above diftances, upon the fuppofition of the 
ftar, that revolves as a fatellite, being very much {maller than the central one, fo 
as not to-difturb its place fenfibly; for if the two ftars fhould contain equal, or 
nearly equal, quantities of matter, the periodical times might be fomewhat lefs, on 
account of their revolving about their common centre of gravity, in circles of 
little more than half as great a diameter as that in which the fatellite muft revolve 
upon the other fuppofition, 
VoL. LXXIV. I ’ 
