TROUGHTON. 133 



jects of interest to Mr. Troughton, and long after his infirmities 

 were an effectual bar to the applications of his most esteemed 

 friends, he exerted himself to supply the seamen with well adjusted 

 and accurate sextants. " Your fancies," he would say, " may wait ; 

 their necessities cannot." 



In 1778 he took out a patent for the double framed sextant, a 

 construction which, combining firmness and lightness, yet admitted 

 of a considerable radius in this invaluable instrument. After trying 

 and rejecting the repeating reflecting circle of Borda, Mr. Troughton, 

 in 179G, hit upon one of his happiest constructions, the British re- 

 flecting circle, as he delighted to call it, an instrument which in 

 right hands is capable of wonderful accuracy. It is a characteristic 

 trait of Mr. Troughton, that in order to bring his favourite circle 

 into general use, he reduced its price far below the usual profits of 

 trade ; and if he had succeeded in his attempt, he might have been 

 ruined by his success, for his sextants were by far the most gainful 

 article of his business. 



With the same earnestness to promote the interests of navigation, 

 he invented the dip sector (afterwards re-invented by Dr. Wollaston), 

 and expended time, money, and ingenuity to no inconsiderable 

 amount, in attempting to perfect the marine top for producing a 

 true horizontal reflecting surface at sea. The marine barometer, 

 the snuff-box sextant, and the portable universal dial, owe to him 

 all their elegance, and much of their accuracy. Where others in- 

 vented or sketched he perfected. 



In the ordinary physical apparatus Troughton made considerable 

 improvement in the construction of the balance, and of the moun- 

 tain barometer. In the same class may be mentioned the form 

 given to the compensated mercurial pendulum ; his pyrometer, by 

 which some very valuable expansions have been determined; the 

 apparatus by which Sir George Shuckburgh attempted to ascertain 

 the standard of weight and measure ; and that apparatus which, in 

 the hands of Francis Baily, has given an invariable simple seconds 

 pendulum. In the ordinary geodesical instruments Mr. Troughton 

 greatly improved the surveying level and staff, and reduced them 

 both in weight and price, with increased convenience and accuracy. 

 It is, however, in the construction of astronomical instruments that 

 this great mechanician particularly excelled ; here he reigned with- 

 out a rival. His portable astronomical quadrants are models of 

 strength and lightness, while the repeating circle of Borda, an 

 instrument which he disliked, first received its beauty and accuracy 

 from his hands. 



The ordinary reading micrometer, and the position micrometer, 

 commonly employed in the measurement of double stars, were 

 greatly improved by him in simplicity and brought to perfection ; 

 and he first applied the former to dividing, though in circles and 

 scales it had already been used in reading off. 



