2S 



easy contrivance to be substituted, where required, instead 

 of this operation on the edge of the mirror ; and which can 

 be executed with httle additional labour by any instrument- 

 maker ; so as to afford a sufficient field of view, with a ca- 

 pacity for accurate adjustment. 



This I have effected by the contrivance of a second small 

 index mirror; requiring only one plane surface, and fixed 

 on the index at right angles to the great mirror ; being totally 

 free and detached from the index mirror, and capable of every 

 adjustment for itself, without interfering with, or impeding 

 any motion requisite for that purpose for the index glass, or 

 altering its position. The following description of such a one, 

 Avhich I have made, will shew that it is a very simple ajid 

 easily fabricated addition to the quadrant. 



image in their interval of separation, will make the observation witli the naked eye un- 

 certain and troublesome) ; it is neccsary that the light from both surfaces of the glass, 

 should be contiguous, having no interval ; which can only be effected, either by making 

 the index glass almost one quarter of an inch thick, or by reducing the thickness of the 

 horizon glass to less than t'jth of an inch; or by such a mutual compensation of both, 

 as would still leave one as much too thick as the other would be too thin, for the uses 

 above stated: and though this may be remedied, while yet the glasses remain of thcit- 

 due and proper dimensions, by using a telescope, whose aperture is large enough to 

 take in botti beams of the reflected light v/itlt the interval of their separation ; yet in 

 ordinary quadrants, of simpler construction and more moderate price, not designed to 

 be furnished with a telescope, or mirror with a polished edge, I cannot but think that 

 an easy and cheap substitution for both, would, if found to answer, be very useful ; as 

 securing at all times the advantages of Mr. Blair's invention for the back observation, 

 (at least for taking altitudes), to those navigators, who do not furnish themselves with 

 a more perfect and expensive instrument ; as well as to those who on land desire, in sur- 

 veys, to ascertain large angular distances by the quadrant ; or by an artificial horizoa 



