j HALLEY'S COMET. 



; the sun, but its changes were sometimes unaccountably rapid : on one occasion 

 it was observed to become obscure and enlarged in the course of a few hours, 

 though a little before, its nucleus was clear and well defined. On the llth of 

 October, the Rev. T. W. Webb and two other observers remarked corusca- 

 tions in the tail. On that evening, at seven hours and thirty minutes, the tail 

 was very conspicuous, extending between x and y Draconis, and evidently 

 fluctuated, or rather coruscated, in length, being occasionally short, and then 

 stretching in the twinkling of an eye to its full extent, which was at least equal 

 to ten degrees. Its changes were extremely similar to the kindling and fading 

 of a very faint streamer of the auroia borealis. 



" The influence of the ethereal medium on the motion of Halley's comet will 

 be known after another revolution, and future astronomers will learn, by the 

 accuracy of its returns, whether it has met with any unknown cause of distur- 

 bance in its distant journey. Undiscovered planets beyond the visible boundary 

 of our system may change its path and the period of its revolution, and thus 

 may indirectly reveal to us their existence, and even their physical nature and 

 orbit. The secrets of the yet more distant heavens may be disclosed to future 

 generations by comets which penetrate still further into space, such as that of 

 1763, which, if any faith may be placed in the computation, goes nearly forty- 

 three times further from the sun than Halley's does, and shows that the sun's 

 attraction is powerful enough at the distance of 144,600,000,000 of miles to 

 recall the comet to its perihelion. The periods of some comets are said to be 

 many thousand years, and even the average time of the revolution of comets 

 generally is about a thousand years ; which proves that the sun's gravitating 

 force extends very far. La Place estimates that the solar attraction is felt 

 throughout a sphere whose radius is a hundred millions of times greater than 

 the distance of the earth from the sun." 



" The orbit of Halley's comet is four times longer than it is broad ; its length 

 is about three thousand four hundred and twenty millions of miles about 

 thirty-six times the mean distance of the earth from the sun. At this perihe- 

 lion it comes within fifty-seven millions of miles of the sun, and at its aphelion 

 it is sixty times more distant. On account of this extensive range, it must 

 experience three thousand six hundred times more light when nearest to the 

 sun than in the most remote point of its orbit. In the one position the sun will 

 seem to be four times larger than he appears to us, and at the other he will 

 not be apparently larger than a star." * 



The appearance of this comet so near the time predicted by astronomers, 

 and in positions so nearly agreeing with those which were previously calcu- 

 lated, is a clear proof of the astonishing accuracy which has been introduced 

 into astronomical calculations, and of the soundness of those principles on 

 which the astronomy of comets is founded. It likewise shows that comets in 

 general are -permanent bodies connected with the solar system, and that no very 

 considerable change in their constitution takes place while traversing the 

 distant parts of their orbits.f 



* Mrs. Somerville's " Connexion of the Physical Sciences," a work which, though written in a 

 popular style, would do honor to the first philosophers of Europe. Of this lady's profound mathe- 

 matical work on the "Mechanism of the Heavens," the Edinburgh Reviewers remark: "It is 

 unquestionably one of the most remarkable works that female intellect ever produced in any age or 

 country ; and with respect to the present day, we hazard little in saying, that Mrs. Somerville is the \ 

 only individual of her sex in the world who could have written it" } 



t The most particular observations on Halley's comet, during its appearance in 1835, which I 

 have seen, are those which were made by the Rer. T. W. Webb, of Tretire, near Ross, an account 

 of which, with deductions and remarks, was read to the Worcestershire Natural History Society. 

 The observations were made with an excellent achromatic telescope, by Tnlley, of 5 feet 6 inches 

 focal length, and 37-10 inches aperture. Through the kindness of this gentleman, I was favored 

 wii'a a manuscript copy of these observations, and would have availed myself of many ol his 

 judicious remarks, had my limits permitted. 



