This superior accuracy of the small instrument requires some explanation. One which has been suggested is, that Captain Sabine employs a chronometer to observe transits with; and that since it beats five times in two seconds, each beat will give four-tenths of a second; and this being the smallest quantity registered, the agreement becomes more probable than if tenths were the smallest quantities noticed. In general, the larger the lowest unity employed the greater will be the apparent agreement amongst the differences. Thus, if, in the transit of stars near the pole, the times of passing the wires were only registered to the nearest minute, the intervals would almost certainly be equal. There is another circumstance, about which there is some difficulty. It is understood that the same instrument,--the thirty-inch transit, was employed by Lieutenant Foster; and it has not been stated that the wires were changed, although this has most probably been the case. Now, in the transits which the later observer has given, he has found it necessary to correct for a considerable inequality between the first and second wires (See Phil. Trans. 1827). If an erroneous impression has gone abroad on this subject, it is doing a service to science to insure its correction, by drawing attention to it. Should these observations be confirmed by other observers, it would seem to follow that the use of a chronometer renders a transit more exact, and therefore that it ought to be used in observatories. Among the instruments employed by Captain Sabine, was a repeating circle of six inches diameter, made by order of the Board of Longitude, for the express purpose of ascertaining how far repeating instruments might be diminished in size:--a most important subject, on which the Board seem to have entertained a very commendable degree of anxiety. The following extract from the "Pendulum Experiments" is important: "The repeating circle was made by the direction, and at the expense of the Board of Longitude, for the purpose of exemplifying the principle of repetition when applied to a circle of so small a diameter as six inches, carrying a telescope of seven inches focal length, and one inch aperture; and of practically ascertaining the degree of accuracy which might be retained, whilst the portability of the instrument should be increased, by a reduction in the size to half the amount which had been previously regarded by the most eminent artists as the extreme limit of diminution to which repeating circles, designed for astronomical purposes, ought to be carried.