Self- Calculating Sextant. 527 



the ordinary means now in use, which may, at any time, be 

 ascertained by the test of experiment. 



It has been objected to Hadley's Sextant, that, by reason 

 of reflex vision, the scale on the circular arc is reduced 

 one-half. In the proposed Sextant, from the same cause, 

 the scale on the line of cotangents is doubled; for what- 

 ever halves the arc, doubles the cotangent; hence the latter 

 not only remedies the objection, but, doubling the original 

 scale, quadruples the chance of accuracy in a comparison 

 with the former. 



Farther, by substituting the measurement of a right line 

 for that of a circular arc, the calculation necessary for a 

 mere transference of terms, is saved. 



The Silurian System. By R. I. Murchison, Esq., 

 F.R.S., F.L.S.i 



In our first notice we reviewed that part of Mr. Murchi- 

 son's work which treated of those deposits which pass from 

 the oolitic beds down to the coal measures. In our second 

 notice we endeavoured to lay before our readers the chief 

 peculiarities of the coal measures, in all cases pointing out 

 any points of coincidence between Indian and English for- 

 mations that our experience may have suggested, thus ren- 

 dering our notice of Mr. Murchison's work of more interest, 

 we trust, than a dry barren criticism. The small coal-field 

 of Newcut, in Gloucestershire, is alluded to by Mr. Mur- 

 chison to show how the regular series of formations may be 

 interrupted, and how the new red sandstone may rest on 

 the old red without the intervention of the coal measures ; 

 and in some cases where even the old red sandstone itself 

 is absent, and the new red thus brought into contact with 



1 Continued from p. 236. 



