In an observatory where, as in that of the Calton Hill, the principal object of pursuit is the determination of the exact places of the fixed stars, and the investigation of those exceedingly slow secular variations, which require many thousands of years to run their cycle,—the stability of the instruments, as a necessary element to the accuracy of the observations, becomes of the extremest importance.

To secure this quality much invention and no little ingenuity have been employed, but not yet with perfect success; for invariably the more accuracy we demand, the more insuperable difficulties appear to arise. Even nature at last appears to be taxed beyond her powers, for we find when we have passed beyond a certain degree of magnifying power, that there are no bodies absolutely stiff and rigid—none constantly of the same dimensions; but all are expanding and contracting, and giving and limiting with every change of temperature or application of small accidental pressures.