234 



KNOWLEDGE. 



June, 1913. 



Geocentric concept of the universe), to their baleful 

 influence ; in later days for the same good reasons 

 the weather conditions of our own earth were 

 supposed to be modified by their presence, and 

 examples of this are to be found in most 

 astronomical works, as well as in our paper 

 "Comets and the Weather" contributed to Symons's 

 Magazine. Donati's comet of 1858 was thus made 

 answerable for the hot weather prevailing that year 

 in England, Biela's comet in 1832 similarly in- 

 fluenced the summer of the "year of reform," whilst 

 the great comet of 1811 caused the ripening of the 

 grapes in some provinces of France, resulting in one 

 of the most wonderful vintages ever known. The 

 same comet of 1811, appearing shortly before 

 Napoleon's disastrous 

 campaign in Russia, is 

 credited with having un- 

 favourable omens drawn 

 from it by the Russians, 

 but here it would appear 

 that the omens were 

 worse for the invading 

 French army than for 

 themselves. If the 

 comet of 1769, which 

 appeared at the time of 

 his birth, is to be re- 

 garded as Napoleon's 

 presiding and protecting 

 genius, this one must be 

 considered as his evil 

 one. The origin of all 

 such notions, as has been 

 well remarked, is to be 

 looked for in the vanity 

 of man and his self- 

 constituted rulers, per- 

 haps more excusable in 

 the days when our 

 little earth was looked 

 upon as the centre of 

 the Universe than in 

 our own, but none the 



less utterly without foundation. The natural 

 desire of the human mind to resolve mysteries was 

 satisfied by the pretended explanations of those who 

 knew no more than their questioners, whilst the bold 

 confession that we are all ignorant and merely as 

 children picking up a few pebbles on the shore 

 whilst the great ocean of truth lies unexplored 

 (Newton) was reserved for later days. Though 

 the encomium of Pope upon Newton 



" Nature and Nature's laws lay hid in night, 

 God said, let Newton be, and all was light," 



is, of course, in excess of the truth and as we have 

 seen, Newton himself did not think so, yet there 

 probably has never: been any work of human genius 

 greater than the incomparable " Principia." Here, for 

 the first time, it was definitely laid down that a body 

 moving under the influence of a central force (such 



as gravitation) varying inversely as the square of 

 its distance from the centre of force, will describe 

 one of the conic sections, an ellipse (circle as a 

 special case), parabola or hyperbola. It had been 

 previously suspected that one or two comets moved 

 in long ellipses or parabolas and Dorfel in 1680 

 showed that the great comet of that year moved in 

 such a path. Newton showed that its motions 

 were entirely in accordance with gravitational 

 principles and the parabolic elements of its orbit 

 were calculated according to methods given by him. 

 This comet approached unusually close to the Sun 

 when nearest, and for a short time must have " been 

 heated to a temperature many times that of molten 

 iron" (Newton). Hallev considered that this comet 



moved not in a para- 

 bola but in a very long 

 ellipse, giving it a period 

 of five hundred and 

 seventy-five years, which 

 led Whiston in his " New 

 History of the Earth" 

 to speculate that it was 

 to an earlier return of 

 this comet that the 



debit: 



had 



Figure 



Comet 



P. J. Melotte and C. R. Davidson, Royal Observatory, 



Greenwich. 1908, Oct. 3 d 9 h 4 m G.M.T. Position of Comet : 



R.A. 20 h 44 m ; Decl. + 68° 58'. Reflector: Ap. 30-in. 



(0-762tn) ; F.L. 11-ft. 5-in. (3-48m). Exp. 30 min. 



Noachian 



been due, and that at a 

 later return it would 

 cause the destruction 

 of our planet by fire ! 

 However, Encke's later 

 calculations, ascribing a 

 period of eight thousand 

 eight hundred years to 

 the comet, are to be 

 preferred to Halley's 

 result, so that Whiston's 

 speculations need not 

 be further considered. 



The three curves 

 known as the conic sec- 

 tions ; parabola, ellipse 

 and hyperbola, as their 

 names imply, may all 

 be obtained by cutting a cone in different ways 

 by a plane, but perhaps they may be more 

 intelligibly defined to the non-mathematical as 

 being obtainable by throwing the shadow of a 

 circular disc upon a plane surface, such as a table. 

 If the disc be held parallel to the table, we get a 

 circle, if obliquely an ellipse, a closed oval curve; if 

 we hold it edgeways to the light we have a straight 

 line. If now we raise the disc so that its highest 

 point is on a level with the source of light, we shall 

 get a parabola, which is oval at one end, but the two 

 sides open out and do not meet again. If now we 

 hold the disc still higher, we shall get another curve, 

 whose two sides will separate even further from the 

 other. This curve is the hyperbola (or geometrically 

 one branch of the hyperbola, there being another 

 similar branch). 



Whilst the planets move in ellipses little differing 



