1837.] Captain Rater's Altitude and Azimuth Instrument. 137 



XI. — Remarks on the use of Captain Rater's Altitude and Azimuth 

 Instrument. — By Lieutenant J. Campbell, t Assistant Surveyor 

 General. 



In the 20th volume of Jamieson's Edinburgh new Philosophical 

 Journal, Mr. Galbraith has given a paper upon the powers and use of 

 Kater's circle, which appears to be the first account of the instrument 

 published. 



After shewing the degree of accuracy with which astronomical ob- 

 servations 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 travel- 

 lers, and to medical officers attached to foreign stations, who are anxi- 

 ous to distinguish themselves in geographical and astronomical re- 

 searches." 



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 ver- 

 tical circle has two verniers which read to i', 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 mag- 

 nify 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 re- 

 ceive 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 com- 

 pared with the circle for portability, and that requires the aid of an 



