Aiko $ 5 u 
} ESSAY ON THE VELOCITY OF LIGIIT. 145 
that it was owing to the uncertainty of the observations, and that either this or the foregoing 
were not so exact as we had before supposed, for which reason we purposed to repeat the 
observation again, in order to ‘determine from whence this difference proceeded, and upon 
doing it, on December 20, I found that the star passed still more southerly than on the former 
observations. This sensible alteration the more surprised us, in that it was the contrary way 
from what it would have been had it proceeded from an annual parallax of the star; but 
being now, pretty well satisfied that it could not be entirely owing to the want of exactness 
in the observations, and having no notion of anything else that could cause such an appa- 
rent motion as this in the star, we began to think that some change in the materials, &c., of 
the instrument itself might have occasioned it. Under these apprehensions we remained 
some time, but being at length fully convinced, by several trials, of the great exactnoss of 
the instrument, and finding, by the gradual increase of the star’s distance from the pole, that 
there must be some recular cause that produced it, we took care to examine nicely, at the 
time of each observation, how much it was, and about the beginning of March, 1726, the star 
was found to be twenty seconds more southerly than at the time of tho first observation. It 
now seemed to have arrived at its utmost limit southward, because, in several trials made 
about this time, no sensible difference was observed in its situation. By the middle of April 
it appeared to be returning back again towards the north, and about the beginning of June 
it passed at the same distance from the zenith as it had done in December, when it was first 
observed. 
‘From the quick alteration of this star’s declination about this time, (it increased a second 
in three days, ) it was concluded that it would now proceed northward, as it before had gone 
southward, of its present situation; and it happened as was conjectured, for the star con- 
tinued to move northward till September following, when it again became stationary, being 
then near twenty seconds more northerly than in June, and no less than thirty-nine secouds 
more northerly than it was in March. From September the star returned towards the south, 
till it arrived, in December, to the same situation it was in at that time twelve months, allow- 
ing for the differenco of declination on account of the precession of the equinox. 
‘«This was a sufficient proof that the instrument had not been the cause of this apparent 
motion of the star, and to find one adequate to an effect seemed a difficulty. A mutation of 
the earth’s axis was one of the first things that offered itself upon this occasion, but it was 
soon found to be insufiicient; for though it might have accounted for the change of declina- 
tion in y Draconis, yet it would not at the same time agree with the phenomena in other stars, 
particularly in a small one almost opposite, in right ascension, to y Draconis, at about the 
same distance from the north pole of the equator; for though this star seemed to move the 
sameway, as a mutation of the earth's axis would have made it, yet (it changing its decli- 
nation but about half as much as y Draconis in the same time, as appeared upon comparing 
the observations of both, made upon the same days, at different seasons of the year) this 
plainly proved that the apparent motion of the stars was not occasioned by a real mutation, 
since, if that had been the cause, the alteration in both stars would have been nearly equal. 
“The great regularity of the observations left no room to doubt but that there was some 
regular cause that produced this unexpected motion, which did not depend on the uncertainty 
or variety of the seasons of the year. Upon comparing the observations with each other, it 
was discovered that in both the before-mentioned stars the apparent difference of declination 
from the maxima was always nearly proportional to the versed sine* of the sun’s distance 
from the equinoctial points. This was an inducement to think that the cause, whatever it 
was, had some relation to the sun’s situation with respect to these. points. But not being 
able to frame any hypothesis, at that time, sufficient to solve all the phenomena, and being 
very desirous to search a little further into this matter, I began to think of erecting an instru- 
ment for myself at Wansted, that, having it always at band, I might, with the more ease and 
certainty, inquire into the laws of this new motion. The consideration, likewise, of being 
able, by another instrument, to confirm the truth of the observations hitherto made with Mr. 
Molyneux’s was no small inducement to me; but the chief cause of all was the opportunity 
L should thereby have of trying in what manner other stars were affected by the same cause, 
whatever it was; for Mr. Molyneux’s instrument being originally designed for observing y 
Draconis, (in order, as I said before, to try whether it had any sensible parallax,) was so 
contrived as to be capable of but little alteration in its direction—not above seven or eight 
minutes of a degree—and there being few stars within half that distance from the zenith of 
Kew bright enough to be well observed, he could not, with his instrument, thoroughly ex- 
amino how this cause affected stars differently situated with respect to the equinoctial and 
solstitial points of the ecliptic. 
‘* These considerations determined me; and by the contrivance and direction of the same 
ingenious person, Mr. Graham, my instrument was fixed up August 19, 1727. As I had no 
convenient place where I could make use of so long a telescope as Mr. Molyneux’s, I con 
tented myself with one of but little more than half the length of his, (viz: of about 124 feet, 
*The versed sine of an arc A B of a circle is the distance comprised between the ex- 
tremity A of the are and the foot of the perpendicular, let fall from the extremity B on the 
radius drawn to the point A. 
10 s 
