250 Dr Brewster's Improvement on the Nautical Eye Tube. 



From the average of these three years, the mean annual range 

 is 87°.5. The mean annual temperature was found to occur 

 nearly at 10 h a. m. 



President Caldwell remarks, " that the annual mean from 

 the observations of 1820 and 1821 is 59° 54. The observa- 

 tions of 1822, he adds, would carry it a little higher, but the 

 accuracy of this last is doubtful." 



Assuming, therefore, with President Caldwell, that the mean 

 temperature of Chapel Hill is 59°.54 



We have the temperature calcu- 

 lated by Dr Brewster's formula, 

 viz. - - T=86.3 sin. D-— 3^-57°. 7 



Difference between the formula and observation, 1°.84 



Art. XV. — Account of an Improvement on the Nautical Eye- 

 Tube* By David Brewster, LL.D. F.R.S. Lond. and 

 Sec. R. S. Ed. 



A description of Mr Adams's nautical eye-tube was com- 

 municated to this society last winter, and was accompanied by 

 certificates of its excellent performance on board the Clio. Mr 

 Adams was not able to illustrate by one of the instruments 

 themselves ; but I have now the satisfaction of submitting to 

 the society a perfect instrument, through the kindness of Mr 

 Copland and Mr Park, of his Majesty's ship the Diamond, by 

 both of whom it has been frequently used in their voyages 

 across the Atlantic. This instrument was ordered by Lord Na- 

 pier for the purpose of submitting Mr Adams's invention to a 

 fair trial. 



The principle of Mr Adams's construction will be under- 

 stood from Plate IV. Fig. 5, where ACBD is the field of view 

 of the sextant telescope, to which the eye-piece is applied. A 

 piece of mirror glass GH, is placed obliquely above a small 

 level at L, so as to reflect the image of the bubble up to EF, 

 immediately above the field of view. Two marks, M, N, are 



* Read before the Society of Arts for Scotland, February 6, 1827. Mr 

 Adams's invention is fully described in this Journal, No. vii. p. 95. 



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