232 



♦ KNOWLEDGE ♦ 



[Oct. 12, 1883. 



like wings irradiated by the rays of the setting sun. They 

 fly without any visible motion of their -wings, and seem to 

 drink in joy and pleasure in the few hours which lie 

 between their appearance and disappearance, their life and 

 death. 



They measure from 17 to 19 millimetres without the tail 

 filaments, which in the female are of the same length as 

 the body, but in the male double the length. 



The larvffi inhabit the water, and have upon each side of 

 the back part of the body six tufts or tassels ; the head 

 runs forward into two points, and has fine hairy feelers ; 

 the legs are smooth, the front ones the strongest, and 

 adapted for digging. They are fond of hiding under stones 

 or burrowing into the sandy shores, and make a very 

 curious tunnel, something like a double-barrelled gun, 

 which is often 52 millimetres deep. — From BreJan's 

 " Animal Life" Scientific American. 



THE FACE OF THE SKY. 



From Oct. 12 to Oct. 20. 

 By F.R.A.S. 



THE telescope should be directed to the Sun on every clear day- 

 The aspect of the night sky may be gathered from Map X. of 

 " The Stars in their Seasons." Mercury is now a morning star, 

 and, about the time of his greatest western elongation from the Sun 

 on the 22ud, may be seen near the horizon before sunrise somewhat 

 to the south of east, by any one who will get up soon, or sit up late 

 enough for that purpose. Venus, for all practical purposes, con- 

 tinues invisible. Mars gets slowly larger ; but his angular diameter 

 only amounts to 8"'8 by the 26th, so that very little in the shape of 

 detail is visible upon his face. He rises about 10 h. 21 m. p.m. on 

 the 2Gth. Jupiter will be in so-called "Conjunction" with Mars 

 on the night of the 19th Jupiter being about 59' to the south of 

 him. As both planets are so close together in the sky, it is needless 

 to add that they are equally indifferently placed for the observer. 

 Saturn, rising sooner and sooner every night, will be above the 

 horizon about Ch. 10m. p.m. by October 26. His superficial 

 details can now be studied vei-y advantageously. Uranus is 

 invisible ; but Neptune, as we said a fortnight ago, may be 

 found as a small 7th mag. star to the S. and E. of ^ Arietis. 

 The age of the moon to-day at noon is 11'3 days and 

 pretty obviously, on the 26th it will be 25'3 days. During the 

 early morning of the 16tli there will be a partial eclipse of the 

 moon. The first contact with the penumbra (or faint edge of the 

 Eai-th's shadow) will occur at 4h. 42'-im. a.m. The first contact 

 with the real shadow, at 5h. oO'lm. a.m., and the middle of the 

 eclipse at 6h. 54'3m. a.m., Greenwich mean time. Before this, 

 though, the moon will have set. If we call the moon's diameter 1, 

 only 0'27", or less than three-tenths of it, will be covered at the 

 time of greatest obscuration. Two occultations only of fixed stars 

 by the moon will occur during available hours within the nest 

 fourteen days. The first is that of the 6th mag. star B.A.C. 1272, 

 which will disappear on October IS at the Moon's bright limb 

 at 7h. 48m. p.m., at an angle of 36° from her vertex, to reappear 

 at her dark limb at 4h. 3om. p.m., at a vertical angle of 

 267°. The second occultation happens on the 21st, when 

 one minute after midnight X Geminorum, a star of the 3^ 

 mag., will disappear at the moon's bright limb at a vertical 

 angle of 55°. It will reappear from behind her dark limb at an 

 angle of 216' from her vertex at 1 h. 2m. a.m. on the 22nd. The 

 Moon will be in Aquarius all day to-day and to-morrow until about 

 8 p.m., when she will cross the boundary into Pisces. It will be 

 four o'clock in the afternoon of the 16tli before she has completed 

 her path across this great constellation, and entered Aries. She 

 will reman in Aries until 4 a.m. on the 18th, at which time she will 

 cross the boundary into Taurus. Her path through this constella- 

 tion will occupy until 1 p.m. on Saturday 20th, when she will pass 

 into the northern part of Orion. She will be until two o'clock the 

 next morning crossing this, and will then enter Gemini. Here she 

 will continue until six p.m. on the 22nd, when she enters Cancer. 

 She will not quit Cancer until 10 a.m. on the 24th, when she crosses 

 into Leo. Some twenty-four hours later she descends into Sextans. 

 About seven a.m. on Oct. 26 she re-enters Leo, and is still crossing 

 that constellation at midnight on the 26th. 



stcasi 



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POXS'S COMET. 



[953] — The comet that was first detected by Bi-ooks, on Sep- 

 tember 2, and subsequently seen by Mr. Wendall, of Harvard 

 College, and also at Lord Crawford's observatory-, Bunecht, was 

 observed by me in laverpool, on the night of October 3 ; its right 

 ascension being 16h. 14m. and declination north 58° 25', therefore 

 it is about 3° 22' south-west of r] Draconis ; from this data its 

 position can easily be determined by reference to Proctor's " Star 

 Atlas." 



Seen through a 30-inch achromatic, with a magnifying power of 

 60, it presented the appearance of a globular vaporous disc, showing 

 a slight degree of condensation towards the centre — no stellar 

 point could be discerned with the power employed. A star (of 

 about the 9th magnitude), which shows no diminution in brilliancy, 

 can be seen shining through the nebulous edge of the cometary disc. 



It is supposed to be identical with Pons's comet of 1812, which 

 has a period of about seventy-two years. W. K. Bradgate. 



SOFTENING WATER. 



[054] — In a guide to the Fisheries Exhibition now before me, I 

 find an allusion to Atkins's process of softening hard water, 

 " which turns lime in solution into a solid bi-carbonate." 



Would you or any one of your intelligent readers kindly inform 

 me of the nature of this process. Lewis James. 



HULL-DOWN SHIP. 



[955] — Referring to the query of " H. A. L. S.," No. 951, page 

 219, I think Mr. Proctor has missed the correct explanation. I 

 have little doubt that the disappearance and reflection described 

 was caused by a layer of air of less — not of 'jreater — density than 

 that above it. It is a well-known law in optics that light cannot 

 pass from a medium of higher into one of lower refractive power at 

 anything beyond a certain degree of obliquity. When this obliquity 

 is exceeded, total reflection occurs, the rarer or less refracting 

 medium is absolutely opaque to the incident rays, though the film of 

 this medium be no thicker than gold-leaf. 



If at the time when " H. A. L. S." made his observation the sea 

 was warmer than the air above, such a film of warmer air would be 

 in contact with the surface of the water, and the rays coming from 

 the hull-down ship would strike the bending horizon surface at the 

 maximum of possible obliquity, and, therefore, suffer total reflection. 

 A portion of the sea itself would thus be hidden and replaced 

 visually by a mirror-like surface easily confounded with the sky 

 beyond. 



This is rarely seen over an unbroken sea horizon, but is an every 

 day (i.e., every sunny day) phenomenon wherever there is a stretch 

 of sea bounded by an horizon of gradually sloping land, especially 

 if that land be rocky and dark coloured, like the glaciated islet 

 rocks on the Arctic coast of Norway. There the air, heated by 

 contact with the warm rock, renders the rock itself invisible up to 

 a certain height, and makes it appear as though suspended in the 

 air. 



This form of mirage is curious in itself, but I have observed 

 something else connected with it that is still more curious, viz., 

 that sailors and fellow-tourists to whom I have pointed it out had 

 never observed it before, though they must have seen it hundreds 

 or thousands of times. In " Through Norway with a Knapsack," 



