Comets--Their small inclinations--Their motions chiefly direct--Comet of 1770 and 1844--Cause of acceleration in the case of Encke--Anomalous motions of the comet of 1843--Change of diameter at different distances of a comet from the sun--Cause of this change--Nature of the nebulosity--Formation of the tail--Compound nature of a comet's light--motion and direction of a comet's tail--Phenomena presented by the great comet of Halley--Mass of a comet--The Zodial light--Nebulous stars--Shooting stars--Periodic showers--Periodicity doubtful--Cause of the apparent periodicity--Cause for being more numerous in Autumn than in Spring. 187 SECTION SIXTH. State of the polar ice since 1845--Sir John Franklin's track--Probable existence of islands north of Behring's Straits--Possibility of subsisting in the Arctic islands--News from the Investigator--Necessity of searching in a higher latitude than the Investigator visited--Franklin's misfortunes due to Scientific Errors--Relative levels of the Atlantic and Pacific Oceans--The Arctic seas more accessible in a few years--Conclusion. 233 PREFACE. On presenting to the public a work of this novel character, overstepping, as it does, the barriers erected by modern systems to the further progress of knowledge, a few words of explanation may not be inappropriate. Early imbued with a desire to understand the _causes_ of natural phenomena, the author devoured with avidity the interpretations contained in the elementary works of orthodox science, until reason and observation rendered him dissatisfied with the repast. To him it appeared that there was an evident tendency in scholastic instruction, to make the knowledge of nature inaccessible to the many, that the world might be made more dependent on the few; while many of the _established principles_, on which the learned rested, seemed to be at variance with the simplicity and consistency of truth. Thus situated, he ventured to think for himself, and looking back on the history of the past, and finding so many cases in which the philosophy of to-day was supplanted by a different system on the morrow, he was led to suspect the possibility of future revolutions, and was thus determined to be no longer embarrassed by previous systems, nor deterred by opinions however learned, which conflicted with a rational recognition of the mechanical nature of all physical phenomena.