Page:Madras Journal of Literature and Science, series 1, volume 6 (1837).djvu/159

1837.]

In the 20th volume of Jamieson's Edinburgh new Philosophical Journal, Mr. Galbraith has given a paper upon the powers and use of Eater's circle, which appears to be the first account of the instrument published.

After shewing the degree of accuracy with which astronomical observations may be made with so small an instrument, Mr. Galbraith remarks: "the deductions from these small but compact instruments are much more accurate, therefore, than from their size we had any reason to expect, and may be advantageously employed in many geodetical and astronomical operations with great success. Hence, the smaller sized ought to recommend themselves to scientific travellers, and to medical officers attached to foreign stations, who are anxious to distinguish themselves in geographical and astronomical researches."

The dimensions of one of these small circles in my possession, made by Robinson, of Devonshire-street, Portland-place, London, are as follows:—

The case is nine inches high and five inches square at the bottom, and, having pieces of brass for the levelling screws let into the top, it serves upon occasion as a stand for the instrument The circles are three and a half inches in diameter, cut to 30′ on a silver arc. The vertical circle has two verniers which read to 1′, and the azimuth circle has three verniers which also read to 1′. The level is fixed to the back of the vertical circle. The telescopes are achromatic and magnify seven times, and the one attached to the vertical circle is fitted with two negative eye-pieces, one of which is diagonal, and a dark glass is adapted to each; this telescope has four wires, one vertical and three horizontal, which are convenient in taking altitudes, because a mean of three observations may be obtained without touching the instrument.

A stand with three legs, like that of a theodolite, with a plate to receive the adjusting levelling screws, with another to embrace the upper part of the screws to hold the instrument firm, would be a convenient support on some occasions, but a small table, or a bullock trunk, makes a very good stand, when no better is at hand.

The pocket box sextant is the only instrument which can be compared with the circle for portability, and that requires the aid of an