5o8 M E M O I R S ^ f^^ 
of the fun and l^ar 5 then by bringing the two oppofite points in 
the fixed circle to the perpendicular, which is done at the time of 
6xing the initrument, move the circle till the day of the month 
come to any of theie, and the ring is re^^tified for that day • and 
if the air be clear, you will lee the fbar ftand at the true time of 
the day or night. 
It may be obje6?:ed, that in a few years, by the annual increafe 
of its declination, the pole-flar will, by moving in a leffer circle, 
be brought too far from the edge of the ring, that the exa6l 
hour and minute cannot well be diftinguifhed ; but this inconve- 
niency, when it is one, may be eafily remedied feveral ways; either 
by making a lefier ring, or by extending a fine thread of filk 
crofs the ring, till it cuts the flar, and at the fame time it gives 
the hour, or, which will yet make this inflrument commodious 
for other purpofes, there may be made an index to move on the 
center of the hour wheel, which being brought to cut the ftar 
■with the edge that proceeds from the center, it will at the fame 
time cut the hour 5 and now we need not be follicitous abqutthe 
exa61 diameter of the ring, provided it do but a little exceed the 
diflance of the pole-flar from the pole, the focal length of the 
glafs being made the radius. 
Mr. rlaniftead has dilcovered, that there is a parallax of the 
earth's annual orbit at the pole liar; of about 40 or 45 leconds^ 
whereby the diameter of the ftar's parallel is greater va June than 
an Tiecemher^ by about i min. 2 ieconds; which he has evinced 
from ieven years lucceffive obiervations, whereby the earth's mo- 
tion is undoubtedly demonflrated: Now if upon the edge of this 
index there be drawn a icale of degrees, minutes, and leconds, to the 
radius of the glals, we fhall have not only a very accurate inftru- 
ment for the hour, but be furniflied with one, whereby we may fee 
the truth of the earth's motion confirmed, by the accefsand recefsof 
our ftar towards and from the pole, according to the earth's place 
in the ecliptic, as Mr. Flamftead has difcovered ; and that not 
only when the ftar tranfits the meredian, but in a clear air at any 
time of the day; one fhall likewife obferve that annual increale 
of the declination of the pole-liar, caufed by the preccfTion of 
the Eqidnos* 
Mr. Gray's obiervations alllircd him, that the pole-ftar may 
be I'een in the day time with a telefcope of 1 5 foot; for with one 
of that length he law it on the 2dth of y^pril 1701 from 
4 o'clock in the morning 'till 7, and could have leen it longer, 
had not clouds interpoled; and again the ill of May, tho' he 
did not look for the ftar, untill the fun had been up more than 
half 
