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THE TRANSIT OF VENUS.

The transit of the planet Yenus, now to be seen close above the position of the sun after sunset, is now the topic absorbing the nations of the world, who have, it seems, sent forth thoroughly equipped parties to its various corners. lii the frozen north, to Siberia, to Russia, and to other parts; in the south, lo the Chatham Islands, to Kergulen Island, to the Auckland Islands, and .-to New Zealand. We may ask what- is all this for? —10. see merely, a black spot pass over the sun, or for what other purpose has all this expense been incurred? It has been undertaken as-a fit'work for the nations of the world to act togethor in —with a view to perfect the lunar tables, or, in other words, to make -the Nautical Almanac as near perfect as possible~to complete or clinch, so to speak, man's knowledge of the solar system. Such is the nature of the work to be begun and finished during some four or five hours on the 9th instant, not only in New Zealand, but we may say over the globe we inhabit. There was never a time in the history of the world when so important a tasklay before astronomers; and when our means (owing to telescopic improvements, excellence of chronometers, and the marvellous electric spark wherewith to flash aud compare chronometers) were so perfect to insure that degree of accuracy so esseutial when dealing with the solar system and the mighty spans that separate its various globes. The dark body of Yenus will be seen to touch the sun at about 25 minutes past. 1 p.m. on Wednesday, the 9th at .the lhames, and may be seen with auy moderate telescope or opera glass, or even by a good eye protected by smoked glass. In a good telescope a black notch or indent will be first noticed, denoting what is termed the first external contact ingress; after a lapse of about 29 minutes Yenus will bo seen as a round black spot, the edges of the sun and Yenus touching, Yenus being within the sun's disc; this denotes first internal contact ingress, and is the first important instant for time. Some three and a half hours will now elapse, during which this black spot will appear to traverse along the suu's face, near the lower part (seen direct), when internal contact egress takes place, the seend important event; 29 minutes later will take place external contact egress. All is now ended—the workis done; and calculations will very soon show whether the earth is 91,000,000 or 95,000,000 miles distant from the sun. The distance of the sun frern the earth may be said to be the basis of all practical astronomy, and enters as an element into every problem connected with the solar system. The transit of Venus, especially such a transit as will take place on Wednesday, is the best means of ascertaining this all-im-portant factor. We understand that a diagram show.ing the relativo sizes of the sun and Venus, path of lh» latter, &c., will be exhibited in Mr Burra's window to-day, and will no doubt prove interesting.

Mr Severn's apparatus for observing the transit are: A 10-feet 8-inches focus reflecting telescope, of 11 inches clear aperture, mounted as an altazimuth, with slow movement in altitude and azimuth. 1 his tolescope is of the "front view" or " Hcrschelian" construction. The tube is built of kauri battens, x finch in thickness, and of the requisite length. These have been built into light iron hoops, of which there are live. The strips of kauri are kept apart half-an-incii by a small wedge of wood immediately under each ring. The whole is thoroughly tight, and the tube so stiff and strong that it could be walked on without in any way deflecling it. About four feet from one end is a strong iron band, secured by a second circle of short battens. This heavy ring is the main' support of the tube when slung. At one end of the tube are three fine steel screws, in diameter. On these three screws are six brass nuts, to be after described. The other end of the tube is partially closed by an iron plate, -g thick On the left-hand side of this plaie is a circular aperture, to admit of the rays or the light from the object to enter thia aperture is 11 inches diameter. Immediately to the right of this stands the <ye-piece, fitted with rack motion. A light wood frame incloses this end of the tube; and on the right hand of it is fixed the movement for slowly raising or lowering the instrument, consisting of a wheel 15 iuches diameter, with so/all brass wire teotb. This is acted upon by a pinion turned by the right hand. The former winds or unwinds a roller, on which is wound the cord that suspends the tube. Under the centre of the frame is a small axis, attached to a small pair of brass bevel wheels. Tho former winds or unwinds the side motion of tho tube, and the small wheel carries the motion near to tho finder, a small telescope wherewith to fiud an object previous to observing it in the large one. The tube is thus mountedA strong post, some 10 inches diameter and 11 iVct high, is placed perpendicularly in the ground, and as firmly fixed as possible, being five feet in tho ground on the top of this, and in its axis is an iron pin pointed at top. A long box, just large enough to enclose the post freely, is now put on the top end, having au iron plato inside to rest on the pin at top of post; thus tbe box revolves freely around tho post. At lower end of box are four friction rollers running on a ring fixed to tho post. A support of iron is screwed to tho revolving box, destined to carry an inch iron bolt through two holes; this bolt is then screwed into the main ring on tubo of telescope. Thus, by means of two sets of pulleys the tubo may be lowered or raised with very little troublo, and it can be mado to revolvo around tho first.; it can be pointed to any part of the heavens. This form of mounting w«s adopted, on account of its economy, as also to at once assist in tho ooiupletion of the glass speculuma or reflector. 'Jhcso wero begun some six years ago, mi aro made in an uuusual manner. J[n

place of casting the glass discs, as Is usually done, a plan was adopted which Mr Severn has found to fulfil the required conditions of the work. The discs were cut out from ordinary plate glass; Iheso were gradually heated to redness on cast iron discs some 3 inches in thickness and turned (approximately) to the convexity required. After careful annealiug, the convex discs were cemented into cast iron cases, having three equally distant lugs on their circumferences, with a view to fixing same on the three steel strews at end of tube. The discs so made; were then operated on by the process of sand and emery grinding, honing, polishing, and figuring, &c. This part of the process is simply indescribable, as it involves the greatest mechauical skill of any process known, as also much chemical and mathematical knowledge-the object, in view being to give the speculum or rector such a form that all parallel rajs of light falling on it shall be reflected and collected at one .point, termed the focus; this point; is situated at a-distance of half • lie radius of the curvature of the reflector. To pass on. When the curve is perfect—which can only be determined by viewing small double stars and such" like test objects under high powers—the glass surface is silvered by a beautiful chemical process, and the speculum of. glass is said to bo finished, In. this process the tools for grinding are made of various forms some of galvanised iron, of pewter, of water of Ayr stone, pitch, resin, &c., and on the completion of such work the polish and the_ curve must finish at one and the same time - "ft good curve or figure to the .speculum is of no use without a good polish, and vice versa. Mr Severn wisely began two such mirrors (duplicate); one is, therefore, always as a stand-by, whilst the other may be improved upon. The machine to do this work, has been working at the Bright Smile engineroom, the room having for some five months been kindly, lent for the purpose. Tophotograph the sun duringtbe transit of Venus, .VJr Severn has made a small camera, some three inches square; this replaces the eye-piece. The sun's image, direct from, the mirror, has a diameter of I*2 inches in the. focus, and when thrown on a collodion film, the solar spots and detail can be plainly seen, and the limb or edge of the sun is jwell defined. To open the' shutter of the little camera (a very delicate operation) at any given instant of time, a revolving shutter has been made; worked by an indianrubber spring around its axis. It has an angular opening in it, which passes by and, on account of its form, equally exposes every part of the prepared plate to the cone or focus of rays -no matter whether the. time of exposure is one second or the two hundredth part of a second. To liberate this shutter an electro magnet is fixed to the top of the camera, which when galvanised liberates a catch which previously held the shutter. It is then merely necessary to get the sun in the fiuder, touch an ivory button or electric key, and the picture is instantly tafcn, and at the same exact instant a one-tap electric bell strikes. This bell will be close to chronom-ters, of which there will be two, tim»d and ratel with all possible care. The telegraph wire is in direct communication with the telescope, to enable Mr Severn to obtain the absolute time from Major Palmer, of the Venus Expedition, .in Christchurch. The arrangements Mr Severn has contrived are so-precise that they would admit of Major Palmer, in Christchurch, taking a photograph of the sun here at any instant during the transit. The whole of the apparatus is constructed in the simplest and most economic manner, and only such materials are used as could be procured on the spot. Close to the telescope accommodation has been made for-some 100 galvanic battery cells, and apparatus from the Telegraph Office, wherewith to. enable Mr Severn and Major Palmer to compare their time, both before and after the transit, by direct instantaneous telegraphic communication. Ihe necessary instruments are in position for this purpose. Accommodation is also made for two chronometers, a one-tap electric bell, and time takers. This electric bell is arranged to strike when a button is touched at the eye piece of telescope; also for a small room in which the photographs will be chemically dealt with. To prepare for a windy day three large screens, 12 x 12, are in readiness. These, if necessary, will be placed to windward of the telescope. On the 9th instant, Mr Severn will be assisted by several friends, who have kindly lent their assistance, distributed thus:—Two taking the time at chronometers; one photographing ; one keoping telescope always on the sun's centre; two looking to telescope ladder and wind screens; and one taking notes of the various appearances presented by, Venus at the important epochs These are exclusive of telegraph operators from the Grahamstown office.

We may state that no visitors will be allowed on the day of transit, because the work to be done is exceedingly difficult, and admits of no intrusion whatever. Mr Severn will, however, show the apparatus to persons interested after 5 p.m. to-day (Saturday), and on Monday. We may state that on Thursday night Mr Severn, Mr Sheath, and Mr Bull communicated with Lieutenant Palmer at Burnham in the most satisfactory manner.

Permanent link to this item
Hononga pūmau ki tēnei tūemi

https://paperspast.natlib.govt.nz/newspapers/THA18741205.2.15

Bibliographic details
Ngā taipitopito pukapuka

Thames Advertiser, Volume VII, Issue 1911, 5 December 1874, Page 3

Word count
Tapeke kupu
2,023

THE TRANSIT OF VENUS. Thames Advertiser, Volume VII, Issue 1911, 5 December 1874, Page 3

THE TRANSIT OF VENUS. Thames Advertiser, Volume VII, Issue 1911, 5 December 1874, Page 3

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