uracy of our results. In a case therefore
like the present, where the base of our isosceles triangle is to the
other two sides as eight units to twelve thousand, it is impossible
not to perceive that it behoves us to be singularly diffident as to the
conclusion at which we have arrived, or rather it behoves us to take for
granted that we are not unlikely to fall into the most important error.
We have satisfied ourselves that the sides of the triangle including
the apex, do not form an angle, till they have arrived at the extent of
ninety-five millions of miles. How are we sure that they do then? May
not lines which have reached to so amazing a length without meeting, be
in reality parallel lines? If an angle is never formed, there can be no
result. The whole question seems to be incommensurate to our faculties.

It being obvious that this was a very unsatisfactory scheme for arriving
at the knowledge desired, the celebrated Halley suggested another
method, in the year 1716, by an observation to be taken at the time of
the transit of Venus over the sun(50).

(50) Philosophical Transactions, Vol. XXIX, p. 454.

It was supposed that we were already pretty accurately acquainted with
the distance of the moon from the earth, it being so much nearer to us,
by observing its parallax, or the difference of its place in the heavens
as seen from the surface of the earth, from that in which it would
appear if seen from its centre(51). But the parallax of the sun is so
exceedingly small, as scarcely to afford the basis of a mathematical
calculation(52). The parallax of Venus is however almost four times as
great as that of the sun; and there must therefore be a very sensible
difference between the times in which Venus may be seen passing over
the sun from different parts of the earth. It was on this account
apprehended, that the parallax of the sun, by means of observations
taken from different places at the time of the transit of Venus in 1761
and 1769, might be ascertained with a great degree of precision(53).

(51) Bonnycastle, Astronomy, 7th edition, p. 262, et seq.

(52) Ibid, p. 268.

(53) Phil. Transactions, Vol. XXIX, p. 457.

But the imperfectness of our instruments and means of observation
have no small tendency to baffle the ambition of man in these curious
investigations.

"The true quantity of the moon's parallax," says Bonnycastle, "cannot be
accurately determined by the methods ordinaril