308 Intelligence and Miscellaneous Articles, 



A shall not tend to disturb the relative place of micrometer B, and 

 that the movement of micrometer B shall not tend to disturb the 

 absolute place of micrometer A (A and B standing in exactly the 

 same relation as the tangent- screw of a mural circle, and the micro- 

 meter in its telescope) : and then the observation may be conducted 

 in this manner. One wire being very nearly in the position for bi- 

 secting the star, A will be read, and the bisection will be completed 

 by B. Without waiting to read the micrometer head, the object- 

 glass, &c. will be reversed, and the second bisection will be com- 

 pleted by A. Then A and B will be read. It will be proper some- 

 times to effect the reversion in the opposite order : and for this 

 purpose, using the same hand on the micrometer, B must be read 

 before beginning, and the first bisection must be completed by A, 

 and after reversion the second bisection must be completed by B. 

 In either case the complete double observation may be obtained 

 with great rapidity, but without the smallest hurry. 



The Astronomer Royal expressed his belief that an instrument 

 thus constructed might be expected usually to give results accurate 

 to one-tenth of a second of arc. 



The Astronomer Royal then stated that, before attaching any 

 name to this construction, he had requested the assistance of the 

 Master of Trinity College, Cambridge, whose authority in a philo- 

 logical question of this kind is undisputed. Doctor Whewell has 

 fixed on the name " The Reflex Zenith Telescope ; " a name which 

 appears to express with singular accuracy the peculiarities of its 

 construction, and which the Astronomer Royal hoped would be 

 universally adopted. 



XL. Intelligence and Miscellaneous Articles. 



ON CARBONATE OF LIME AS AN INGREDIENT OF SEA-WATER. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, September 6, 1849. 



1 N the current Number of the Philosophical Magazine appears an 

 *- abstract of a paper by Dr. Davy On Carbonate of Lime as an 

 Ingredient of Sea-water, from which, according to the author's in- 

 ferences, it appears that "carbonate of lime, except in very minute 

 proportion, does not belong to water of the ocean at any great di- 

 stance from land." But with all deference I would submit that this 

 may be true as regards the surface only ; and that the bottom of the 

 sea, even at its greatest distance from land, may be equivalent to 

 " proximity to coasts," the point urged by Dr. Davy. 



That there are grounds for such a supposition, and for believing 

 that no inconsiderable quantity of carbonate of lime does exist at the 

 bottom of the ocean far from land, is apparent in Darwin On Coral 

 Reefs, where instances are given of living corals and corallines being 

 taken up from great depths ; and taking the presumption that the 

 base of some of the remote coral islets of the Pacific, whose perpen. 



