296 



THE POPULAR EDUCATOR. 



but few comets were observed. Now, however, scarcely 

 :i year passes without four or five being 1 observed, and 

 frequently the number is greater. For the most part, how- 

 ever, they are so small as only to be visible by the aid of 

 good telescopes. If is only 

 at rare intervals that those 

 large ones which are at 



period is the longest of all thov.e whose orbits are fully ascer- 

 tained and verified by subsequent returns. 



On its appearance in 1682, just after attention had been 

 drawn to the phenomena of comets by the appearance, in 1680, 



of a brilliant one, whose 



once seen by the unaided eye 

 become visible to us. The 

 periods of these likewise are, 

 as a rule, very much longer 

 than those of the telescopic 

 ones, so that only the orbita 

 of a few have been determined. 

 The first indication of a comet 

 is usually a faint luminous 

 speck visible with a telescope. 

 This appears gradually but 

 slowly to increase in size aa 

 the comet approaches the sun, 

 and soon a bright spot known 

 as the nucleus is discerned 

 in it. This ia usually of a 

 circular form, and situated 

 nearer to the side which ia 

 directed towards the sun. In 

 telescopic comets this nucleus 

 is not always discernible. 



As the comet approaches 

 the sun, it becomes larger 

 and brighter, the coma or 

 cloudy mass around ths 

 nucleus also becomes lesa 

 regular, and soon a tail 

 begins to be thrown out on 



the side remote from the sun. This it is which forms the 

 most remarkable feature in the appearance of a comet. The 

 tail is usually more or less curved, and points away from the 

 sun, so that when receding from that body the comet travt 

 with its tail foremost. The 

 annexed figure (Fig. 29), 

 which represents the comet 

 of 1811, gives a good idea 

 of the general appearance 

 of these bodies, the nucleus, 

 coma, and tail being all 

 distinctly marked. A period 

 of 3,065 years was assigned 

 to this comet. On their re- 

 turn after completing their 

 orbits, comets seldom pre- 

 sent the same appearance as 

 before ; hence they cannot 

 be identified by their form, 

 but only by the calculation 

 of their orbits. According 

 to many old illustrations, 

 some comets have presented 

 very remarkable shapes, at 

 times closely resembling 

 swords or sabres. Allow- 

 ance must, however, be 

 made in these for the ima- 

 gination of the artist excited 

 by the terror occasioned by 

 the appearance of these 

 bodies. A few comets have 

 had more than one tail. 

 One, visible at the end of 



Fig. 29. COMET OP 1811. 



1823, had in addition to 

 the usual tail a second 

 one directed towards the 



sun. The comet of 1744 is, however, the most remarkable, 

 as it is stated that when it approached the sun the tail was 

 divided into six distinct branches, all curved in the same direc- 

 tion, and extending 30 or 40 in length. 



Halley's comet is one that has attracted as mnrh attention 

 as any, as it was the first whose orbit was calculated, and its 



motion Sir Isaac Newton had 

 investigated, Halley carefully 

 examined its movements, so 

 as to ascertain whether those 

 of any which had previously 

 been noted would in any way 

 accord with them. He soon 

 found that in several respects 

 it seemed to resemble those 

 of 1531 and 1607, and ima- 

 gined that all three might be 

 in reality appearances of the 

 same body, its period being 

 somewhere about 75^ years. 

 This conjecture proved to be 

 correct, and Halley's comet 

 is now regarded as one cf 

 the members of our system, 

 revolving round the sun in 

 a period of 76'78 years, its 

 greatest and least distances 

 from the sun being 3,200 and 

 f>6 millions of miles respec-- 

 tively. The return of this and 

 all other comets is frequently 

 retarded or accelerated ly 

 the attraction of planets 

 which happen to lie near 

 their course. The period 

 given above is, however, tl.e 



mean, and on its last return in 1835 the allowances to be made 

 for the influences of the planets were so carefully calculated, thai 

 the date of its perihelion passage was predicted within four days. 

 s i The next return of this body will take place in the year 1912. 



On searching back through: 

 the lists of comets that 

 have been seen, very many 

 appearances of this body 

 can be traced ; one as far 

 back as 11 B.C.; and the 

 comet of 1066, already re- 

 ferred to, was, doubtless, 

 another appearance of the- 

 same body. 



In 1456 it appeared with 

 a very long tail considerably 

 curved, and an eclipse of the 

 moon occurred when the 

 comet was in close proximity 

 to it, creating intense alarm. 

 The comet of 1858, known 

 as Donati's, will be remem- 

 bered by many. Fig. 30" 

 represents its appearance 

 on the 3rd of October in 

 that year. Its brightness 

 was, however, greater than 

 shown there. 



It was first seen on the 

 2nd of June, at Florence, by 

 Dr. Donati, after whom it is 

 named. Its movements for 

 the next two months were 

 vrry slow. Towards the end 

 of August faint traces of a 

 tail began to be seen, and it 



Fig. 30. DONATI'S COMET. 



soon became visible to the 



naked eye. It accomplished its passage round the sun on the 

 29th of September, its tail vastly increasing in length, being on 

 the 10th of October upwards of 50,000,000 miles long. 



On the 5th of October the comet passed in front of the star 

 Arcturus, and though a portion of the tail at its densest part, 

 having a thickness cf several thousand miles, intervened 



