Note on the Self- calculating Sextant. 225 



inconvenient for a portable instrument. From the general 

 description, it would seem that the instrument much resem- 

 bles that called the Surveying Square, which is a square with 

 sight fitted to one side, and which has a moveable rule or 

 index also bearing sights, which turns on a pivot at one cor- 

 ner. On two sides a scale of equal parts is laid off, by which 

 the proportional parts of a plane triangle can be roughly esti- 

 mated, and by a plumbline or level attached to the index, ver- 

 tical plane triangles can be also estimated in the same manner. 

 Angles are measured in the most accurate instruments 

 now used by a circular scale of equal parts; because it 

 is easily divided, and being equi-distant from the centre, 

 admits of the application of the verniers, while a tangental 

 scale of equal parts cannot be read off with any degree of 

 accuracy, in consequence of the varying angle between the 

 tangent and the secant. 



Observations on the Fossiliferous beds near Pondicherry, and 

 in the district of South Arcot. — By C. T. Kaye, Esq., 



Madras Civil Service. 



The existence of abed of fossiliferous limestone in the neighbourhood 

 of Pondicherry has long been pretty generally known to those who take 

 an interest in such subjects, and some attention was recently attracted 

 to it by several communications in the Spectator Newspaper. An ob- 

 servant person, indeed, can hardly fail of being struck with the nature of 

 the stones which form the paving of some of the streets and the steps of 

 many of the houses in Pondicherry, and which are replete, not only 

 with the fragments of innumerable shells, but with many ostreas and other 

 bivalves almost as entire as if they still reposed in their proper ele- 

 ment. The silicified wood at Trivacary is also well known to the pub- 

 lic from the beautiful polish which it receives, and from its adaptation 

 to table and other ornaments. To the geologist it is interesting from 

 the vast size of the petrified trees, (one of which is nearly 100 feet 

 long,) from their great number, and from the perfect state in which 

 these organic remains of other worlds are preserved. 



It had long been a desideratum to collect data on which the era and 

 nature of the formations which contain the fossils might be decided, and 

 I accordingly took an opportunity of leisure in October last, to proceed 



2 G 



