282 Extract of a letter J'rom Dr Turnhull Christie 



those days), repeat the former operation, using the sun's de- 

 clination corresponding to the 1st July in the computation of 

 the altitudes ; do the same at all the other lines, and through 

 all the points thus determined draw the hour lines 2 E, 3 F, 

 4 G, &c. and the dial will be completed. 



Proof. — The line AB is made to coincide with the line AH, 

 or the visible horizon in the construction ; let S represent tlte 

 sun, then the angle SBH is equal to the sun's altitude. But 

 the angle SBH is equal to the angle ABE, to which AE is 

 tangent, at the radius AB. But it is evident from the figure 

 that AE must be the length of the gnomon's shadow when the 

 sua is at S, and as the point E is the tangent of the sun's alti- 

 tude at 2 o'clock, the contact of the point of the shadow at that 

 line must be the hour required. 



Extract of a letter from Dr Turnhull Christie to Professor 

 Jameson regarding the Bone Caves of Palermo^ ^c. 



My Deae Sir, Palermo^ %\st May 1831. 



JN OTwiTHSTANuiNG all the classical interest of Italy, I saw 

 nothing in it that gave me more real pleasure and satisfaction 

 than Vesuvius, which I saw to great advantage. To a geolo- 

 gist, his first view from the summit of that beautiful mountain 

 is one of the greatest pleasures he can expect to enjoy in this 

 life, and I saw it to perfection. It was exactly in such a low 

 state of activity as to enable us to walk down into the great 

 crater, and round the summit of the internal one, which was 

 thrown up in December last year, and which is at present only 

 emitting smoke, and occasionally a few ashes. It has now an 

 elevation, I should think, of about 300 feet above the bottom 

 of the large crater ; and the highest point of the mountain, 

 which is on the north edge of the latter, I calculated, by a 

 single barometrical observation, in connection with one made at 

 the same time at Naples, to be 3677 feet above the level of the 

 sea. 



Hitherto I have looked upon my journey merely as one of 

 pleasure. It is from this place that the real business of my 



