_First_, That the Theories and Discourses here collected, have already past the Censure of the _Learned World_: Who have acknowleg'd them the most satisfactory Accounts of Nature's Proceedings, wherein some of her greatest Depths are fathom'd, and a Foundation laid for Posterity to build an infinite _Superstructure_. _Secondly_, That they are related (_Verbatim_) just as they were delivered in, or read before the _Royal Society_: For it has been the Opinion of the most Judicious among those _Honourable Members_, that it is impossible so to abridge them, (which are but Abridgments themselves) as not to render them obscure and unintelligible. A TABLE OF THE _Discourses_ contain'd in this _Volume_. _An Estimate of the Quantity of _Vapours_ raised out of the Sea, as derived from Experiment: Together with an Account of the _Circulation_ of the wat'ry Vapours of the Sea, and of the Cause of _Springs_. Presented to the Royal Society by Mr. _E. Halley_, F. R. S._ Pag. 1 _The True Theory of the _Tides_, extracted from that admired Treatise of Mr. _Isaac Newton_, Intituled _Philosophiæ Naturalis Principia Mathematica_; being a Discourse presented with that Book to the late King _James_, by Mr. _E. Halley_._ 13 _A Theory of the _Variation_ of the _Magnetical Compass_, by Mr. _E. Halley_._ 27 _An Account of the Cause of the Change of the _Variation_ of the _Magnetical Needle_, with an Hypothesis of the Structure of the _Internal Parts_ of the _Earth_; as it was presented to the Royal Society in one of their late Meetings, by Mr. _E. Halley_._ 43 _An Historical Account of the _Trade-Winds_ and _Monsoons_, observable in the Seas between and near the _Tropicks_; with an Attempt to assign the Physical Cause of the said Winds, by Mr. _E. Halley_._ 61 _A Discourse of the Rule of the Decrease of the Heighth of the _Mercury_ in the _Barometer_, according as Places are elevated above the Surface of the _Earth_; with an Attempt to discover the true Reason of the rising and falling of the _Mercury_, upon Change of Weather, by Mr. _E. Halley_._ 81 _A Letter from Mr. _Isaac Newton_, while Professor of the Mathematicks in the University of _Cambridge_; containing his new _Theory_ about _Light_ and _Colours_: Sent from _Cambridge, Feb. 6. 1671/72._ in order to be communicated to the Royal Society._ 97 _A farther Explanation of the same _Theory_._ 114 _A Demonstration concerning the _Motion of Light_, communicated from _Paris_._ 118