PLANETARIUM. 



IM.AXT1NQ AND PLANTATIONS. 



Sts 



THEORY, MOTIONS, PERTURBATIONS, INEQUALITY, Ac. 

 Tb Una planet WM Bnt used a* one of distinction between the stars 

 wkieh |^IIITI their planes, or Mam to do to, tad those which have a 

 ssMble motion, tad. a* U now known, about the mm. The etymology 

 would oblige u* to include comeU many of the stars, which hare 

 Mil motions of their own, or proper motion*, M they are railed all 

 Ifcose double stars which revolve about each other and all the satel- 

 lites which revolve about other planets. A* discovery proceeds, it a 

 lilrly that every body in the universe will be found to be planetary. 

 The word however has changed it* meaning, and U used to ttand for 

 a h<*rrnly body which make* successive revolution* about our *un. 

 It thm include* the old planet*, Mercury, Venus, Man, Jupiter, 

 Saturn (excluding the tun and the moon, the moat easily discovered 

 planets of all) the planet of Copernicu* and Galileo, on which wo lire 

 the new plant u or ASTEROIDS, of which there are now (November, 

 1MO) sixty- two; and the periodic comets of Halley, Encke, Biela, 

 Brorsen, D* Arrest, and probably many others. 



The plan of this work leaves us little to say under so general a term 

 as Planet, and that little consists mostly of references, and explana- 

 tions of isolated points. The order of discoveries connected with the 

 object, so far as it is matter of history, must be looked for umler 

 ASTBOXOMT. It is impossible to separate the history of one part of 

 astronomy from that of another : the fixed stars, on which all celestial 

 measurements depend, must accompany the planets in every account 

 of the Utter ; the mtthrtnit.kil and physical considerations which give 

 rise to our power of predicting the motions and places of the planets, 

 apply equally to those of the moon and comets. 



By a Planetary Theory was once meant any hypothesis which serves 

 to explain the motions of a planet, as actually perceived. Thus there 

 WM one theory of Mercury, another of Venus, ftc., each (without 

 connection with the rest) expounding the nature and magnitude of all 

 the supposition* which must be superadded to that of the simple 

 circular motion round the sun, in order to eunble the theorist to 

 predict the planet's place at any given future time. At present, by 

 the theory of a planet is meant the deduction of its motion from the 

 theory of gravitation. Given the places of all the bodies of the solar 

 xystem st any one moment, together with their velocities and the 

 directions of their motions, the assumptions of the theory of gravita- 

 tion are found sufficient to deduce tables which almost rival observa- 

 tion itself in the accuracy with which predictions can be made ; for 

 an instance, see MOON. When tables are constructed with such 

 fundamental data only, they are called theoretical : but if some of 

 those details which should have been deduced from the theory of 

 gravitation have been deduced from observation, the tables are 

 then partly m/aricot For the treatment of the theory of gravita- 

 tion a* a question of physics, see ATTRACTION : for the deduc- 

 tion of the character of the main inequalities of the planetary motions, 

 and most points connected with the detailed results of that theory, 

 see GRAVITATION : for the results which are particularly connected 

 with the secular inequalities, and the reasons which we have to infer 

 the permanence of the whole system, unless acted on by some new and 

 external cause, see SOLAR SYSTEM. See also the general considerations 

 uii'ler the word THEORY. 



The Planetary Theory, it must be remembered, in the sense in 

 which those words are now understood, explains only the motions of 

 the planets round the sun. A spectator situated on the last-named 

 body would find it sufficient to predict the place among the stars, of 

 every planet at every time : or a terrestrial astronomer might assign 

 by it the places of the planets as seen from the sun, which are called 

 ktlieetnine places. But our own circumstances, as being ourselves 

 revolving about the sun, cause the apparent planetary motions to vary 

 most materially from the real ones, and the geocntric places (or places 

 M seen from the earth) to differ literally lot** <; /> from the helio- 

 centric places. Thus, as seen from the sun, a planet moves from west 

 to east, always : ss seen from the earth, it sometimes moves from east 

 to west. As to this point, the circumstances of our day are, in regard 

 to the astronomical education of the world at huge, a complete reversal 

 of those which preceded the time of Newton. Formerly, the apparent 

 motions were well known to those who knew anything; the real 

 motions were matter of dispute : now, every well educated boy of 

 fourteen ha* a very good notion of the real motions, while few, except 

 astronomers, could distinctly explain the apparent ones, and maps of 

 them are quite out of date. To explain these motions here would 

 require us to introduce the content* of another article ; the whole of 

 what is necessary on this point will be found in TROCHOIDAL CURVES, 

 thepreliminary considerations being found under MOTION. 



The places of the principal planets are usually given in the 'm"iv. 

 at intervals of several days, In a manner which is sufficient to find 

 them in the heavens. In the ' Nautical Almanac ' is to be found a 

 Owenwteh meridian ephemeris of every planet for every clay in the 

 year, in which the places are predicted to the smallest quantities, so 

 that the reduced observation of any one day affords on immediate 

 comparison of the theoretical table* with the fact. 



PLANKTAKIl-M. roWMT.l 



PLANING MACHINERY. 6f late years the increase in the price 

 of hand labour, and the necessity for securing mathematical accuracy 

 in the surfaces of contact of the part* of machinery, have led to the 

 application of other machinery to the preparation of the surfaces of 



wood, of atone, and of iron-work. The nature of the various materials 

 has led to modifications of the tools employed, as might naturally have 

 been expected; and it is worthy of remark that the application of 

 machinery to this purpose has been much more successful with the more 

 apparently intractable and hard materials, stone and iron, than it has 

 been with the softer one, wood. Generally speaking, the tools used for 

 planing wood, or iron, are fixed at a certain height, but are able to 

 reverse the direction of their points ; and the surfaces to be operated 

 upon may be moved horizontally under the tools, so as to be attacked 

 in their alternate motion, or only in one direction when the cutting 

 tools are not susceptible of being reversed. In wood planing machinery 

 the majority of the tools hitherto used in England have a rotary 

 motion ; and from this fact, and from the tendency of the wood to tear 

 up, as the workmen say, it is necessary to finish the brat class of 

 joiner's work by hand labour. It may be also stated that until the 

 introduction of planing machinery for iron, it was almost impossible to 

 obtain a large and perfectly level surface on a plate of that metal ; so 

 that in fact thin machine has been one of the most important agents in 

 the advance of the arts connected in any way with the use of iron. 

 The success of the application of machinery to the dressing of stone, or 

 of wood, depends, however, solely upon the value of hand labour ; and 

 its introduction becomes a mere question of local economy, whence its 

 more general use in America. 



In the iron planing machinery there is a large bed or plate connected 

 with the driving machinery in such a manner as to allow the bed to move 

 alternately to and fro in a horizontal direction ; and the tool or chisel is 

 fixed in a carrier, so as either to reverse the position of it* point when 

 the bed has reached the end of its course, or it is raised on a horizontal 

 pin so as to allow of its being lifted by the projections of the surface 

 operated upon, and thus only to work whilst the bed U travelling in 

 one direction. The distance between the bed and the carrier is regu- 

 lated by a screw; and the width of the face of the tool is varied 

 according to the greater or lesser quantity of iron to be removed, the 

 finishing tool being wider than the roughing point. A slight burr is 

 left on each side of the path of the tool, which is removed subsequently 

 if a perfectly smooth face should be required. Almost all the tool 

 makers of the present day manufacture these beautiful machines ; but 

 those made by Mr. Whitworth are certainly amongst the best, even if 

 they be not the very best, of their class. 



Mr. Hunter took out, some years since, a patent for a modification 

 of the iron planing machinery, which has been applied with such 

 remarkable success by the late Mr. L. F. Carnegie, in his flag stone 

 quarries near Arbroath, that it must remain a matter of surprise that 

 the use of the machine has not been more generally diffused. In Mr. 

 Hunter's stone planing machine the bed moves under a chisel, of a 

 width of face varying inversely with the amount to be removed, as in 

 the case of iron planing engines, and the slight burrs are worked down 

 by hand. The American stone planing machines seem to consist for 

 the most part of rotary cutters of chilled cast iron which wear away 

 by friction the surfaces exposed to their action. 



The ordinary descriptions of wood planing machines consist of 

 cutters, or steel plates, working with a rotary motion ; but, as was 

 before stated, they do not leave the finished surface in the state 

 required for good work. The best American machines of this kind, 

 however, have cutters like an ordinary plane iron, screwed to a carrier 

 able to be adjusted over the board to be operated upon by means of a 

 screw. In front of each cutter is placed a loose bar as near to the 

 knife as possible, for the purpose of keeping the grain of the wood 

 perfectly close immediately before the knife. For the best description 

 of joiner's work the wood thus planed requires to be finished off with 

 the smoothing plane, as in the case of the wood prepared by the ordi- 

 nary jack, or trying planes used in ordinary hand labour. In the latter 

 caa-j the rough work is done by the jack plane ; it is carried to a 

 higher ;.oiut by the use of the trying plane, and is finished by the 

 smoothing plane. 



PLANISI'HKRE. This term originally stood for any representation 

 of all or part of the sphere on a plane ; it is now out of use, at least in 

 that sense. It has partially however been retained (and it would be 

 desirable that this meaning should become fixed and general) to signify 

 any contrivance in which plane surfaces moving on one another fulfil 

 any of the uses of a celestial globe. 



The instrument which we proceed to describe is one which when 

 made correctly, at or near a given latitude, is, for ordinary .uses, that 

 is, for finding the position of the heavens at any one moment, much 

 more easily used than the celestial globe, and very much less expensive. 

 A circular disc of pasteboard, on which the stars visible in our latitude 

 are laid down, turns on a second disc, round which are the days of the 

 year on one circle, and the hours of the day on another. A third and 

 hollow disc turns upon the same pivot, the hollow part being so cut 

 that the portion of the heavens which it shows is precisely that which 

 is visible at one time in the latitude of the instrument : the points of 

 the compass are marked round the rim of the hollow disc, or of the 

 horizon. The effect is, that by setting the disc which contains the 

 stars to the day of the year, and the hollow disc to the hour of the day, 

 the part of the heavens visible at that day and hour is distinctly 

 shown. The time at which any star rises, culminates, and sets can be 

 immediately found within a minute. 



PLANTING and PLANTATIONS. Planting is the operation of 



