Mr Adam's Description of a Nautical Eye-Piece. 95 



before it has been long exposed to the air) it emits a strong 

 disagreeable smell. It also appears to contain particles of 

 iron pyrites. 



In Ross-shire this is an abundant rock ; but I have not 

 found it about Inverness in beds of more than three or four 

 feet in thickness. 



These particular remarks are offered, from the considera- 

 tion that the frequency of bitumen in the stony materials of 

 the globe, is becoming more and more an object of interesting 

 speculation. In the enumeration of the general substances 

 containing this ingredient, as detailed by the Right Honourable 

 George Knox, in his excellent paper on this subject, which 

 has been published in the Transactions of the Royal Society 

 of London, no substance is mentioned which exactly corres- 

 ponds to the rock, the characters of which I have described. 

 The nearest approach to it is the occurrence (mentioned by 

 him) of bitumen in mica-slate, and fetid quartz. 



Akt. XVIII. — Description of an inverting Sextant Telescope 

 with Nautical Eye-Tube, for taking Altitudes at Sea when 

 the Horizon is Invisible. Invented by Mr Matthew 

 Adam, A. M. Rector of the Academy of Inverness.* Com- 

 municated by the Author. 



This telescope, represented by AB, Plate I. (Fig. 12.) con- 

 sists of three parts; viz. 1st, The eye-tube AE, to the lower 

 side of which a spirit-level kx is attached by the the screws o, 

 p, passing through the extremities C, and D, of the frame of 

 the level tube ; 2d, The object tube FB, which is attached 

 to the sextant by the screw at y ; and, 3d, The middle, or 

 connecting tube EF, represented separately by GH, (Fig. 13.) 

 of which the part EH enters the object tube at F, and the 

 part EG is screwed into the eye tube at E by means of the 

 screw EK, and thus brings the small glass G into its proper 



* This eye tube was executed under Mr Adam's inspection in July 

 1825, by Mr Robinson, Mathematical Instrument Maker, at 38, Devon- 

 shire Street, Portland Place, London. The dimensions of ihe telescope 

 and level are reduced one-half in the annexed diagrams. — See this Journal, 

 vol i. p. 179. 



