IS] 3.] for a Travelling Astronomer. 103 



Secondly : an artificial horizon of quicksilver, with a glass 

 roof that folds up, to prevent the wind disturbing the surface of 

 the mercury, when the observation is to be made : there should 

 also be a floating glass, ground truly parallel, to place on the 

 surface of the quicksilver in windy weather, or in those places 

 where the ground is liable to be shaken with carriages. 



Thirdly : a pocket compass. The whole of these instruments 

 may be readily packed up in a small leathern case, furnished 

 with straps, to buckle over the shoulders similar to a knapsack. 



The next instrument, and without which the others would be 

 comparatively of little use, is a chronometer : a silver one would 

 be the cheapest, the price being about forty guineas; and I 

 would recommend Mr. Pennington, of Camberwell, to be the 

 maker; and it is better to give the order at least twelve months 

 before it is wanted, as chronometers accelerate when first made, 

 and sufficient attention, generally speaking, is not paid to the 

 compensation for cold ; the same remark is frequently applicable 

 to the want of adjustment when the chronometer is placed in 

 different positions. 



To the instruments already mentioned, may be added a 

 telescope, furnished with a micronometcr for observing the 

 eclipses of the sun and moon, occultations of the stars by the 

 moon, and the eclipses of Jupiter's satellites. 



I am, my dear Sir, sincerely yours, 



M. Bkaufoy. 



Article VII. 



On Transition Rocks. By James Giierson, M. D. 



(To Dr. Thomson.) 

 silt, 



In the last Number of your ylnnals, I read, with much 

 interest, a paper entitled " Remarks on the Transition Rocks of 

 Werner, by Thomas Allan, Esq. F. R.S.E. In tin's paper, 

 after paying some well-merited and proper compliments to the 

 ingenuity and perseverance of Werner, in having furnished us 

 with " means by which rocks may be described with some degree 

 of accuracy, " Air. Allan observes, that " we cannot extend our 

 unqualified approbation to the systematic arrangement he has 

 introduced." Werner, he says, was confined within certain 

 limits by the narrowness of his fortune and the circumstances of 

 his situation, and therefore could not be expected to form a 

 perfect system, and has drawn " conclusions that are more 

 general than are warranted by the circumscribed field to which 

 he was confined." 



I 



