Dec. 2, 1881.J 



KNOWLEDGE 



87 



rations were made on the evening of June 30, nineteen 

 days after it had passed its point of nearest approach to 

 the sun. We remember well ol)ser\ ing it on the 

 morning of July 2, 18G1. For some reason, we 

 found it impossible to sleep tliat morning, and getting 

 up about three in the morning (the exact hour we 

 do not remember, but it must have been very early), we 

 saw in the east what looked at first like the rays of an 

 aurora borcalis. But presently we noticed that these rays 

 proceeded (unlike tliose of the aurora) from a bright centre, 

 which had been hidden by clouds wlien our observations 

 began. We used at that time to keep a four-inch telescope, 

 mounted on a three-legged stand, in our bedroom. This 

 we had quickly re^idy for action (noting that tlic object, 

 owing to the approach of sunrise, was getting fainter every 

 minute), and turning it on the comet, we drew a picture of 

 the nucleus and coma so closely resembling that which 

 appeared a week or two later in the lUustrated London 

 News, that we might have supposed our picture had been 

 surreptitiously sent to the office of the Illustrated, had we 

 not found it resting just where we had put it in our 

 scientific portfolio. 



path almost at right angles to the plane of the ecliptic. 

 Thus the comet could be tracked on its retreat until, finally, 

 distiince concealed it from our \ iew. 



Now, the tail of the comet of 1861, as seen iii Fig. 2, 

 had something of the fan-like expansion observed in the 

 tail of the comet of 1744 ; but what was known of the 

 comet's position at the time when this fan-like form was 

 seen, explained the peculiarity, and showed the necessity 

 of taking into account the position of a comet before at- 

 taching undue importance to the apparent figure of its tail. 

 For the fan-like form seen on this occasion was a mere 

 effect of perspective. The end of the tail appeared very 

 much wider than the part near the head— not that it reall}- 

 was so, but simply l)ecausc it was very much nearer to the 

 observer on earth. When we were actually immersed in 

 the tail, the part nearest to us, being all round, had, to all 

 intents and pui-poses, an infinite extension. But even when 

 the comet was beyond that position, or a few days earlier, 

 before it had reached it, the end of the tail was much 

 nearer to us than the comet's head, and thus appeared far 

 more proportionately widened than was actually the 

 case. 



Such considerations must always be taken 

 into account in dealing with cometic pheno- 

 mena. Comets, more than any other celestial 

 ibjects (the Milky Way, regarded as a whole, 

 I leing, perhaps, alone excepted), are affected in 

 shape, and apparently, even in their very 

 )iatiu-e, by position, and consequent foie 

 shortening. 



The comet appeared to the eye as shown in Fig. 2. Sir 

 John Herechel, who observed it at Collingwood, in Kent, 

 icniarked that it was far more brilliant than any comet he 

 hud ever seen, not even excepting those of 1811 and 18.58. 

 ■Jhe Padre Secchi, at Rome, found that in the clear skies 

 of Italy the tjiil was fully 118° in length, corresponding to 

 nearly one-third more than the distance between the horizon 

 and the point overhead. This comet, by tlie way, though 

 only favourably visible for a very .short time, I'einained 

 witliin the range of telescopic vision much longer. Hind 

 remarks that the number of separate oljservations for the 

 (It-termination of its orbit exceeds 1,150, and extend over a 

 period of Hi months. It travelled on a course favouring 

 observation, coming from remote distances south of the 

 plane in which the earth travels to the northern side of 

 that plane — and as it chanced, crossing the plane (about five- 

 sixths of the way from the sun to tlje earth's orbit) just 

 when the earth lay in the same direction from the sun, so 

 that for a time she was within the Ijounds of the comet's 

 tail-like appendage — and then travelling northwards on a 



SOLIDS, LIQUIDS, AND GASES. 



By W. Mattieu Williams. 



TART III. 



''pHAT the solid and liquid states of matter 

 JL are not distinctly and broadly separable, 

 !iut are connected by an intermediate condi- 

 tion of \ iscosity, which is more or less common 

 ro both, has, I think, been sufficiently shown 

 in the previous papers, and the proofs of this 

 ire familiar enough. 



We now come to the question whether 

 there is any similar continuity between 

 liquids and gases. Ordinary experience de- 

 ci ledly suggests a negati\e answer. We can 

 point to nothing within easy reach that has the properties 

 of liquid and gaseous half-and-half ; that stands between 

 gases and liquids as pitch and treacle stand between solids 

 and liquids. 



Some, perhaps, may suggest that cloud-matter — London 

 fog, for example — is in such an intermediate state. This, 

 liowever, is not the case. White country fog, ordinary 

 clouds, or the so-called " steam " that is seen assuming 

 cloud-forms as it issues from the spout of a tea-k(!ttle or 

 funnel of a locomotive, consist of minute particles of 

 water suspended in air, as solid particles of dust are also 

 suspended. It ha.s been called "vesicular vapour," on the 

 supposition that it consists of minute vesicles, like soap- 

 bubbles on a very small scale, but this hypothesis remains 

 unproven. London fog consists of similar particles, var- 

 nished with a delicate film of coal-tar, and interspersed 

 with particles of soot 



In order to clearly comprehend this question, we inust 

 define the difference between fluids and gases. In the 

 first place, they are both fluids, as already agreed. What, 



