Nov. 11, 1881.] 



♦ KNOWLEDGE 



27 



appear. The latter was the case with the comet of 

 1835-3G, as we have already seen. On the other hand, the 

 comet of 18G1 burst upon us in its full splendour after 

 peri/ielion-TpsiSSAge. 



Some comets have more than one tail. One appeared in 

 17-1-1 which had no less than six tails, symmetrically dis- 

 posed (if one can trust the pictures handed down to us) in 

 the tigure of a half-opened fan (Fig. 2). Others have 

 presented a yet more peculiar appearance, having, besides 

 a tail in the usual position, a second " unconformable " 

 tail, at right>angles to the first, or inclined to it at some 

 incongi-uous, outof-the-way angle — for instance, in one 

 case, one hundred and sixty degrees. Sometimes the pecu- 

 liarity is presf-nted of a perfectly dark gap separating the 

 Uiil from the liead. More commonly a dark space is seen 

 liehind the head, but on each side of this space the light 

 from the head is continued so as to form a bright border 

 on each side of the tail. 



away from the sun. Tlie same sun which attracts the 

 nucleus seems to repulse the emitted matter with 

 inconceivable energy. Consider for a moment what 

 took place with Newton's comet in 1680-81 (Fig. 3). 

 When this comet was about as far oft' from the sun as 

 our earth (ninety million miles) it began to throw out a 

 tail. But the comet was going far nearer to the sun than 

 this. Onwards it rushed under the powerful influence of 

 the sun's attraction, until it had crossed the whole space 

 of ninety million miles, making — almost in a straight line 

 — for a point only one hundred and thirty thousand miles 

 from the sun's surface. In four weeks it traversed that 

 vast distance, and then, suddenly (in a few hours) sweeping 

 half round the sun, started on its return journey. But 

 note this : as it approached the sun, the comet had thrown 

 out a tail continually increasing in length, and pointing 

 back almost along the orbit; then the comet is lost to 

 sight for a few days, and when it is next seen returning 



Fig. 4.— Comet of 1843. 



As a comet approaches the sun, we have seen that a 

 change takes place in the appearance of the coma and 

 nucleus, and that in some instances a tail is generated. 

 The process actually observed is generally this : in the 

 forward part of the nucleus a turbulent action is seen to be 

 in progress, leading to the propulsion towards the sun of 

 jets or streams of misty-looking matter. Sometimes a 

 regular cap or envelope is seen to be projected in this 

 manner towards the sun, or even a set of envelopes one 

 within the other. The matter thus thrown off is not 

 suffered to pass very far from the nucleus towards the sun, 

 but is swept away, as fast as formed, in the contrary 

 direction. If the funnel of a steam-engine were directed 

 forwards, instead of upwards, then the appearance pre- 

 sented by the emitted steam, as .the engine rushed on 

 (against a hurricane, suppose, to make the illustration 

 more perfect) would exemplify the process which seems to 

 be taking place aroimd the front of the nucleus, and far 

 behind it, as the matter formed is continually swept 



rapidly from the "sun, it has a tail jiomtiDg forwards (a tail 

 which muH be a'diflerent one, since— as Herschel says— 

 " we cannot conceive a comet's tail to be brandished round 

 like a stick "), and ninety million miles in length. So that, 

 whereas the comet, already moving with a tremendous ac- 

 quired velocity, had taken four weeks in traversing a distance 

 of ninety millions of miles under the sun's attraction, 

 the matter composing the tail had been thrown to the 

 same enormous distance by the sun's repulsion in scarcely 

 one-tenth part of the time, possibly (for the tail was 

 formed when first seen) in a few hours I 



The comet of 1843 (Fig. 4) was yet more remarkable 

 for the dimensions of its tail and for its close approach 

 to the sun. The tail of this comet stretched half-way 

 across the sky in March, 1843. Its real length was two 

 hundred million miles at least, for the end of thetailwas 

 lost to view through the excessive faintness of its light 

 So near did this comet pass to the sun, that many as- 

 tronomers chd not expect ever to see the comet again. 



