Mr Galbraith on a New Pocket^Box Circle. 



rallel to the volcanic line of the Sneefield-Syssel. The most 

 northern of these valleys is that of Norduraae, and then follow 

 to the south those of Thuraae, Huitaae, and Reikoltsdal. All 

 these valleys indicate by their parallelism a similar origin ; and 

 the cause which produced them is probably to be found in the 

 volcanic trachytic mountain chain of the Sneefield-Syssel. In 

 the Nonduraae valley, which lies farthest to the north-east, we 

 are not a little surprised by finding a volcanic cone of eruption, 

 from which a stream of lava has flowed, for in all other places 

 the trap rocks exhibit no volcanic eruptions. Probably this is 

 the only point in the whole island where a volcanic chimney is 

 to be found among trap hills. 



The southern valleys do not possess an inferior degree of in- 

 terest, from the numerous hot springs to which they give rise. 

 The Reikholtsdal is the most important, and owes its name to 

 the pillars of smoke which it evolves. At its bottom there is a 

 long line of hot springs, stretching over a space of two miles. 

 Many of them exhibit periodical eruptions, and project the wa- 

 ter to considerable heights. All the springs contain silica in 

 solution, and give out sulphuretted hydrogen mixed with the 

 vapour. Silica forms incrustations round their edges. 



A New Pocket-Box Circle* By WILLIAM GALBUAITH, Esq., 

 M. A. Teacher of Mathematics, Edinburgh, M. S. A. 



IN the early ages of the world, the instruments for making 

 astronomical observations being very rude, the results obtained 

 by them were far from accurate. In the days of Hevelius and 

 Flamsteed, large sectors of many feet radius were employed ; 

 and, though divided to a very great degree of precision, yet 

 being only parts of the circle, did not possess that accuracy 

 which might have been obtained by employing the whole circle. 

 At a much later period, quadrants were used at Greenwich, 

 Oxford, and in most of the continental observatories ; and, 

 from the accuracy of their construction by Graham, Bird, 

 Ramsden, and Troughton, important results for the advance- 



Read before the Society of Arts for Scotland, 2Cth March 1836. The 

 Silver Medal, value ten sovereigns, awarded 7th December 1836. 



