110 



SCIENCE-GOSSIP. 



[May ], 1866. 



by the way, are scarcely more pugnacious than 

 caddises themselves. The deception was so perfect 

 that they mistook it for an uncased grub, and, with 

 mouths watering in anticipation of the delicious 

 morsel " so naar and yet so far," dashed against the 

 transparent armour in dire indignation. After a 

 little while they ceased all attacks, I suppose I must 

 say from instinct, though I am far more inclined to 

 call it reason. Caddis-worms can be obtained from 

 almost every river and stream, and as they always 

 build in accordance with the specific gravity of the 

 water, the larger cases will be found in the bed of a 

 deep or swiftly flowing river. These are generally 

 constructed of small pebbles and Planorbis shells 

 (not unfrequently with living inmates), while in 

 shallow streams, or crawling among the river-weeds, 

 may be found those which construct their homes 

 entirely of vegetable substances. I think the 

 caddises might be classed into vegetarians and 

 carnivorians, homoeopaths and allopaths, for some 

 live entirely on a vegetable diet, others prefer meat 

 in large quantities, while some build for days, and, 

 as far as I can see, never eat at all. The carnivo- 

 rians are decidedly preferable for architectural 

 purposes ; they seem stronger and more active ; 

 while the vegetarians use the weeds given them to 

 allay their hunger in building a fresh case. When 

 dredging for caddises, I sometimes transfer from the 

 net to the can the closed-up cases which are every- 

 where to be met with. These contain the pupa?, 

 which pass some time in a quiescent slate. From 

 April to June some of these are sure to emerge, 

 and swim freely about the water for an hour or two. 

 The pupa much resembles the larva, though the 

 colour is brighter, and the eyes distinctly visible. 

 They swim with great rapidity, as far as I could 

 observe, only using the anterior pairs of legs, the 

 hinder ones, which are very long, being folded 

 together along the under side of the abdomen. In 

 a few hours the pupa casts its skin, and the perfect 

 insect {Phri/ganea grandis) will be seen drying its 

 wings on the rock-work or weed. Should it be 

 wished to save the insect, it will be better to cover 

 the aquarium with net as soon as the pupa emerges, 

 in order to secure it before it tries its wings. The 

 eye and wing are both beautiful objects for the 

 microscope. M. Pope. 



OBSERVATIONS ON VENUS. 



rpiIE dazzling brilliancy of this planet makes 

 ■*- the daytime preferable for observing it; but 

 under the best of circumstances its light is far too 

 glaring to permit physical observations being con- 

 veniently made. J. D. Cassiui attacked it in 

 1667, aud some ill-defined dusky spots, seen on 

 various occasions during April, May, and June, 

 enabled him to assign 23h. 16m. for its axial ro- 



tation. Biauckini in 1726 and 1727, favoured by 

 an Italian sky, observed spots with greater facility. 

 He inferred a rotation performed in 21 days 8h. 

 Sir William Herschel, desirous of arriving at a sound 

 conclusion, devoted much care to the inquhw, but he 

 was unable to assign a precise period, or to do more 

 than suspect generally that Bianchini's was largely 

 in excess of the true amount. Schroter, by closely 

 watching certain spots, deduced a period of 23h. 

 21m. 7'98s., which Di Vico and his colleagues at 

 Home, in 1840-2, only slightly modified to 23h. 21m. 

 23'93s. We may thus feel assured that the axial 

 rotation of this planet is known to within a very 

 small fraction of the whole amount. 



Sir W. Herschel' s opinion of the spots he saw- 

 was that they were in an atmosphere, and did not 

 belong to the solid body, an opinion wanting in 

 analogy, and there is now reason to believe altogether 

 groundless, for Di Vico found the spots just as 

 delineated by Bianchini. The Roman observers, six 

 in number, displayed great diligence, and, with one 

 exception, Bianchini's drawings were confirmed. Of 

 the six observers the most successful were those w r ho 

 had most difficulty in detecting very minute com- 

 parisons to large stars, the reason of which Webb 

 points out to be obvious enough. A very sensitive 

 eye, which would detect the spots more readily, 

 would be more easily overpowered by the light of a 

 brilliant star, so as to miss a very minute one in its 

 neighbourhood. 



Mountains probably exist on Venus, though the 

 testimony on which the statement must rest is not 

 quite so complete as could be desired. In August, 

 1700, La Hire, observing the planet in the daytime 

 near its inferior conjunction, perceived in the lower 

 region of the crescent inequalities which could only 

 be produced by mountains higher than the moon. 

 To the same effect, Derham writing in 1715. 

 Schroter asserted the existence of several high 

 mountains, in which he was confirmed by Beer and 

 Madler ; but his details as to toises must be accepted 

 with great reserve, amongst other reasons, because it 

 is doubtful whether his micrometers were of suf- 

 ficient delicacy. Sir W. Herschel disbelieved him 

 on some points, and assailed him in the Philosophical 

 Transactions for 1793. His reply was published in 

 the volume for the year but one after. It was calm 

 and dignified, and indicated the mountains, if not 

 the measurements. Di Vico, at Rome, in April and 

 May, 1841, appears to have noticed a surface-con- 

 figuration akin to that of the moon ; a bluntness of 

 the southern horn, referred to by Schroter, was also 

 seen by the Roman astronomers, and often by Breen 

 subsequently, with the Northumberland telescope at 

 Cambridge. 



That Venus has an atmosphere is generally ad- 

 mitted ; that it is of considerable density is likewise 

 an opinion apparently well founded. During the 

 transits across the sun of 1761 and 1769, the planet 



