92 



• KNOWLEDGE ♦ 



[Aug. 1, 1884. 



bath, accumulate there as dirt. This dirt has been 

 analysed with, as might be expected, very various results. 

 Max Duke, of Leuchtenberg, found a sample to contain 

 tin to the extent of 33-5 per cent., cop|ier 9 4, oxygen 

 24-82, antimony 9 22, arsenic 7 2, silver 4-45, sulphur 

 2-4G, nickel 2 2G, &c. The presence of this dirt forbids 

 the immersion of the mould to the bottom of the batL It 

 should, in fact, be always well off the bottom. One of the 

 objects in view in making a hole at the bottom of a hollow 

 mould, such as the one referred to in the previous article 

 ■when speaking of the elastic mould method, is to allow the 

 flow of the liquid to wash out any dirt that may have 

 fallen to the bottom, and which would otherwise injuriously 

 affect the electrotype. 



The temperature of the solution is another important, 

 although apparently insignificant, detail. If it is too low 

 the decomposition of the solution and the deposition of the 

 metal are hindered. The temperature should never be 

 allowed to fall below 60° Fahr., but, on the other hand, it 

 should not get much above this [loirit. 



In the case of single-faced medallions and such-like 

 objects, when the dimensions are small, the backing of lead 

 may frequently be dispensed with, and a backing of shellac, 

 pitch, or some such material substituted. 



With this, I think I may safely leave this branch of the 

 subject. Of course, in describing the process adopted in 

 taking copies of large statues it was not my supposition 

 that the amateur would attempt such work. My purpose 

 was rather to make clear to him the fact that smaller and 

 less ambitious tasks might be attempted with every prospect 

 of success, and that with ordinary care excellent results 

 might be accomplished. My next effort will be to explain 

 the processes to be adopted in plating or typing leaves, 

 flowers, insects, and other more or less delicate objects. 



VENUS IN A THREE-INCH 

 TELESCOPE. 



By a Fellow of the Koyal Astronomical Society. 



THE glorious planet we are going to examine today 

 surpasses, under certain circumstances, every object 

 in the sky in lustre ; and hence the poet in saying that, 



Hesperns that led 



The stan-y host rode brightest, 



simply expressed a bald matter of scientific fact. About a 

 month after she has attained what is called her greatest 

 elongation east, or the same time before she acquires her 

 greatest western elongation, she may be detected with the 

 naked eye in the sunlit sky; and, when in the former phase, 

 casts a very perceptible shadow at night upon any white 

 surface. Her great brilliance under these conditions 

 renders her the most severe test of the achromatism of a 

 telescope that we possess ; and an instrument must be 

 perfect indeed that will exhibit an absolutely colourless 

 image of her at this time. 



In order that we may have an intelligent idea of what 

 we are going to look at, it will be necessary to recall a few 

 elementary facts in connection with the orbits of the Earth 

 and Venus. Everybody (at least, everybody who will read 

 these lines) knows that Venus goes round the 8un in an 

 orbit inside our own ; in other words, her mean distance 

 from our mighty centre of light and heat is GG^ millions 

 of mUes, while ours is 92?j millions. She travels through 

 this orbit in 224 7 days. Now, if we were standing still, 

 she would go through all her phases in this period, and if 

 she were in, say, inferior conjunction (t e., in a line between 



the Earth and the Sun) on any given day after 2247 dayp, 

 she would return to the same spot. But the Earth 

 itself goes round the Sun in 36.3-20 days, of course in the 

 same direction as Venus, so that what is called her synodic 

 period (Greek ouiocor, a meeting), or time elapsing between 

 one meeting with the Earth and the next is really .383-92 

 day.s. For example, Venus was in inferior conjunction with 

 the Sun at 2 a.m. on the 12th inst. Her next inferior con- 

 junction will not happen until 7 p.m. on February ISth, 

 1886. Now, if we supjjose her to be in inferior con- 

 junction, and also in or near one of the nodes of her 

 orbit, it is pretty evident that she will pass across the 

 face of the Sun as viewed from the Earth, and we shall 

 have a transit of Venus. With this phenomenon, how- 

 ever, we have but small concern here. It last happened 

 on Dec. 6, 1882, and will not recur until June 7th, 2004, 

 when the hand that pens these words and the eyes which 

 rest U])on them will alike be dust and ashes. If, though, 

 the planet is far from her node at the time of inferior 

 conjunction, then she pas>es above or below the Sun as 

 seen by us. On the 12th of July she was nearly 5^ south 

 of the Sun's centre. Under these circumstances, as we 

 shall presently see, while nearly the whole of her lighted 

 face must be turned towards the Sun, yet an extremely- 

 narrow portion of her illuminated limb is perceptible. A& 

 she travels to the westward of the Sun after this as a 

 morning star, more and more of the lighted part of her disc 

 Viecomes %'isible ; until she assumes the appearance of the 

 Moon when in her first quarter ; or, technically speaking, ia 

 " dichotomized." As will be seen by any one who will draw 

 a diagram or plan of Venus's orbit, her diameter must 

 appear the largest at the time of her inferior conjunction, 

 and must diminish just as her illuminated surface increases. 

 After attaining her greatest elongation west of the Sun 

 (which can never exceed 47" 15'), the planet appears to 

 begin to move back again, or from west to east, grows- 

 smaller and smaller, and when her disc is becoming fully 

 illuminated, disappears behind the Sun in the glare of his 

 light, as merely a rather big star. She is then said to be in 

 superior conjunction. Emerging, after an interval, from 

 his rays to the east of him, she becomes an evening star, and 

 goes through all her phases in the reverse order, increasing 

 in diameter as the area of her illuminated surface dimi- 

 nishes. Attaining her greatest eastern elongation, and then 

 turning back as a rapidly- narrowing crescent, she finally 

 returns to inferior conjunction again. This all being, 

 understood, we will, at last, go to the telescope. 



Fig. 1.— Tenns, May 10, 18S-1. Power ICO. 



At p.m., on May 2, Venus had attained her greatest- 

 elongation (45° 27') east, and eight days later the drawing 

 above was made, with a power of ICO, in a 3-in. telescope. 

 Now, two or three things will strike the observer who 

 will carefully scrutinise this sketch. Perhaps the first 

 wUl be the great brilliance of the illuminated limb o£ 



