IV CONTENTS. 



PAGE 



XI. The " Taconic Syatem,^^ and its Position in Straligraphic 



Geology. By Jules IMarcou 174 



XII. Contributions to the Botany of North America. By Asa 



Gray 257 



XIII. Notes on some Species of Gymnosporanyium and Chrysomyxa 



of the United States. By W. G. Farlow 311 



XIV. Contributions to American Botany. By Skreno Watson . 324 



XV. An Examination of the Standards of Length constructed by 



the Societe Ge'nevoise. By W. A. Rogers 079 



XVI. A Method of Filtration by Means of Easily Soluble and 



Easily Volatile Fillers. By F. A. Goocii 390 



XVIT. Observations of Variable Stars in 1884. By Edward C. 



PlCKERI>fG 393 



XVIII. A Photographic Study of the Nebula of Orion. By Edward 



C. Pickering 407 



XIX. Relation between Superficial Energy and Thermo-Electricity. 



By Charles Bingham Penrose 417 



XX. On the Separation of Titanium and Aluminum, icith a Note 



on the Separation of Titanium and Iron. V>y F. A. GoocH 435 



XXI. Atmospheric Electricity. By Alexander McAdie and 



Austin L. McRae 448 



XXII. The Effect of Temperature on the. Magnetic Permeability of 

 Iron and Cobalt. By John Trowbridge and Austin 

 L. McRae 462 



XXIII. Photography of the Infra-Red Region of the Solar Spectrum. 



By William II. Pickering 473 



XXIV. Methods of Determining the Speed of Photographic Exposers. 



By William H. PIckering 478 



XXV. Principles involved in the Construction of Photographic 



Exposers. By William II. Pickering 483 



