It was for this reason that Pond set up a pair of mural circles, one on the east side of its supporting pier and the other on the west.[3] His plan was not only to have each star observed simultaneously in the two instruments, a plan by which, at the cost of some additional labour, he would have got rid, to a large extent, of the individual errors of the two separate instruments, inasmuch as, on the whole, it might have been expected that the errors of the two instruments would have been very nearly equal in amount, but of opposite character. The differences, too, between the two instruments would have afforded the means for tracing these small errors to their respective causes, and so ascertaining the laws to which they were subject. [3] The second circle was intended for the Cape Observatory, but Pond obtained leave to retain it. In 1851 it was transferred to the Observatory of Queen's College, Belfast. Pond went further still. He added to the mural circle a simple instrument, the extreme value of which every astronomer recognizes to-day--the mercury trough. Not only was the star to be observed by both circles when the two telescopes were pointing directly to it, it was also to be observed by reflection; the telescopes were to be pointed down towards a basin of mercury, in which the image of the star would be seen reflected. The mercury being a liquid, its surface is perfectly horizontal; and, since the law of reflection is that the angle of incidence is equal to the angle of reflection, it follows that the telescope, when pointed down toward the mercury trough, points just at as great an angle below the horizon as, when it is set directly on the star, it points above it. If the circle, therefore, be carefully read at both settings, half the difference between the two readings will give the angular elevation of the star above the horizon. A combination, therefore, of all four observations, that is to say, one reflection and one direct with each of the telescopes, would give an exceedingly exact value for the star's altitude. The conception of this method gives a striking idea of Pond's thoroughness and skill as a practical observer, and it is a distinct blot upon Airy's justly high reputation in the same line that he discontinued the system upon his accession to office.