TO CORRESPONDENTS. 



The following Papers have been received : 



Mr. Webster's Communication on the Causes of the Variation of the Magne- 

 tic Pole. 



Mr. Wiseman's Remarks on the Article by Mr. Gutteridge in our December 

 Number respecting the Measurement of Timber : also a Letter from Mr. Paul 

 Newton relative to the same subject. 



The two following Works are in the Press, and are expected to appear in 

 April, each in one Volume, small 8vo, 



A FAMILIAR INTRODUCTION to CRYSTALLOGRAPHY, 

 including an Explanation of the Principle and Use of the Common 

 and Reflective GONIOMETERS, Followed by an Appendix, containing 

 the Methods of applying Mathematical Calculation to the Determination 

 of Crystalline Forms : and Rules for drawing the Figures of Crystals ; with 

 a List of the primary Forms of Minerals, and some Observations on 

 Mineralogical Arrangement, Illustrated by nearly 400 Wood Cuts, or 

 Diagrams. 



By HENRY JAMES BROOKE, F.R.S. F.L.S., &c. 



The System of Notation used by Mr. Brooke, to designate the Planes 

 of Crystals, wil] be adopted in the Third Edition of the following Work ; 

 thus rendering the two Works illustrative of each other. 



AN ELEMENTARY INTRODUCTION TO MINERALOGY, 



By WILLIAM PHILLIPS, F.L.S. M.G.S.L. & C. Member of the 

 Cambridge Philosophical Society. 



This Edition will be illustrated by above 200 New Engravings on 

 Wood, of the Crystalline Forms of Minerals, to be accompanied by the 

 Measurements of their Planes, chiefly taken by means of the Reflective 

 Goniometer. 



This day is published. Price 2s, 6J. plain, or is. 6J. highly finished in 



Colours, 



NO. 30, of ZOOLOGICAL ILLUSTRATIONS, or ORIGINAL 

 FIGURES and DESCRIPTIONS of New, Beautiful, or Interest- 

 ing ANIMALS ; selected principally from the Classes of Ornithology, 

 Entomology, and Conchology. 



By WILLIAM SWAINSON, F,R.S, F.L.S. Member of the Werne- 

 rian Society, &c. 



The Publishers beg to announce, that One Half of this Work is now 

 completed. The unqualified praise that it has received, from those best 

 able to judge of its merits, sufficiently bespeaks its value ; whether as re- 

 garding the truth and beauty of the Plates, or the " original and discri- 

 minative" scientific knowledge evinced in the Descriptions. The 60th 

 Number will terminate the whole ; and die Work will tlien form Five 

 handsome Octavo Volumes. 



Sold by Baldwin, Cradock, and Joy, Paternoster- Row. 



