Mr Galbraith on a New Pocket-boa? Circle. 229 



.ago, proposed to Trough ton, the idea of forming a small pocket- 

 box circle, similar to the one which I now produce, and in which 

 very little of the diameter is lost in forming the radii to which 

 the verniers are attached. Indeed, had the circular box been 

 formed into a sextant instead of a circle, having an index carry- 

 ing a single vernier, the length of that index would not have 

 greatly exceeded those of the circle, on account of the space 

 required for centering, and the segment cut off by the divided 

 arc, and consequently little advantage would by that means be 

 gained from greater length of radius. 



Being unable to induce Troughton to enter into my views, 

 I then attempted to get the late Captain Kater to patronize it, 

 but without effect. I admit the good qualities of Troughton 

 and Dollond's circles for many purposes, though I am also 

 aware that their bulk, weight, and high price, are to many in- 

 superable objections. I am also willing to grant to Kater's 

 small portable circle all its merits as a travelling instrument at 

 land, and I at this time possess one of the very best that Ro- 

 binson has hitherto constructed, having both its horizontal and 

 vertical circles six inches diameter, with three verniers, each 

 showing 10", and from the experience I have had, it promises 

 to give, when properly managed, very accurate results. My 

 object, however, was, by means of the new pocket-box circle, 

 to furnish observers with a very convenient small portable in- 

 strument that might answer every useful practical purpose at 

 sea as well as on land, when accompanied by one of the simplest 

 and most convenient artificial horizons. 



By the able assistance of Mr John Adie, I believe I have 

 succeeded in constructing this little circle in a satisfactory man- 

 ner, though in some of the minor details it may yet be simpli- 

 fied and improved, and I shall feel much pleasure should it 

 meet with the good opinion of this Society, and shall grate/ully 

 adopt any suggestions of its intelligent members for accom- 

 plishing its future improvement, and bringing it more nearly 

 to such a degree of perfection as it may be susceptible of. The 

 three verniers read each to minutes only, but as observations 

 must always be taken, from the nature of its construction, twice 

 once forward and once backward a mean of six readings 



VOL. XXII. NO. XL1V. APRIL 1837. Q 



