176 



♦ KNOWLEDGE ♦ 



[Aug. 29, 1884, 



directs the tube L M at a given hour on any night to a 

 certain star, and notices what division of the card circle, 

 H K, falls opposite the mark on the upright. At the same 

 hour on the next night, he will scarcely notice any appreci- 

 able change ; but if he repeat the observation a fortnight 

 later, he will find that he has to turn the card circle round 

 about as much as for an hour's diurnal motion — that is, 

 through about 71° — before '^L M points to the star. He 

 will notice, however, that L M has not to be turned on its 

 own pivot ; in other words, he will see that the star's 

 distance from the pole of the heavens (the point to which 

 F G is directed) remains unchanged. 



He may confirm this result by a second observation. 

 Let him direct L M towards a star, and in any convenient 

 manner clamp the axis F G, so that L M will remain fixed 

 in position. Then at the end of fourteen or fifteen days 

 let him come to the instrument an hour earlier in the 

 evening than it was set. He will find L M pointing directly 

 towards the star. 



He finds this to be true of every visible star in the 

 heavens, except five. Consequently, all the stars retain 

 the same relative positions among themselves, except those 

 five, which he will therefore look upon as wanderers, a word 

 synonymous with the term planet given to these five stars 

 by the Greek astronomers. Our observer will, for the pre- 

 sent, however, confine his attention to the other stars, 

 which he will call for distinction's sake the fixed stars. 



He now knows that these stars have a somewhat swifter 

 daily motion than the sun, insomuch that in half a mouth 

 the stars have gained an hour's motion on the sun. It 

 follows that in a year they have gained twenty-four hours' 

 motion, or one complete rotation. 



Now here we shall avoid any reference to the diflicult 

 processes by which the exact length of a year is determined. 

 We will suppose that our observer is satisfied by noticing 

 the perfect regularity of the stellar rotation, not only from 

 hour to hour, but from day to day ; and that by some 

 means or other he enables himself to measure time with 

 such exactness that he could detect any apparent departure 

 from the observed regularity of motion, should any such 

 result seem to follow from his excursions over the earth's 

 surface. We shall see that this knowledge, combined with 

 his knowledge of the sun's altitude at noon of every day, 

 or even only with his knowledge of the position of the pole 

 of the heavens, round which the rotation seems to take 

 place, will enable him to form the most certain and con- 

 clusive opinions as to the 6gure of the earth on which he 

 lives. 



On one point he has already gained new information. 

 He has noticed that the star-groups retain their configura- 

 tion altogether unchanged in whatever part of the sky they 

 may be seen. And comparing the observations of one part 

 of the year with those of another, he is enabled to see that 

 the stars are strewn over the sur- 

 face of all that portion of a sphere 

 having E P as axis which can rise 

 above the horizon circle S e N w 

 — in fact, over every part of a 

 complete sphere S ic P K, except 

 the small segments S P' K. Now, 

 at any moment the observer can 

 only see half of the sphere S w P K. 

 He knows that unless the stars, 

 when below the horizon, take up relative positions very 

 different from those they have when visible, they must at 

 every instant cover the portion S w N K of space beneath the 

 horizon, and as he sees no sign of any such change when the 

 stars are visible, while at every season of the year he sees the 

 known star-groups unchanged in aspect, he is clearly justified 



in feeling very certain that no such change takes place after 

 the stars have gone below the horizon. Hence he is certain 

 that the earth is not only limited in the direction of the 

 horizon, but in every ditection below the horizon (except 

 possibly towards the tr "nout S P' K, about which he has 

 as yet no certain inforiijation). He will naturally infer 

 that a complete sphere around him is bedecked with stars, 

 and not a sphere wanting such a segment as S P' K ; but 

 whether this be so or not, he is quite certain about the 

 earth being limited towards all points below S tv N K, 

 except points in S P' K,_/'or he knovjs t/cat stars are streion 

 towards all such points. Knowing that the earth has limits, 

 and perhaps even suspecting already that the earth, which 

 lies within the celestial sphere, is more likely to be in ro- 

 tation than the sphere itself, he sets out to explore his 

 abode. He will direct his explorations first towards the 

 north, to see what changes, if any, may be perceived in the 

 position of the apparent pole of the heavens. Then he will 

 return and travel southwards ; and, lastly, he will make a 

 series of explorations towards the east and towards the 

 west ; untU, finally, the secrets of the earth's figure shall 

 have been completely mastered. 



OUR SUPPLY OF COAL. 



AT a time when approximate calculations have been, 

 made of the date at which our British coal supply 

 will be exhausted, the discovery of a reserve of some 

 8,000,000 tons must possess great popular, as well as more 

 purely scientific, importance. Hence a pricis of a lecture 

 delivered before the members of the Cotteswold Field Club 

 on the 12th instant, by Mr. Handel Cossham (for which 

 we are indebted to the Bristol Afercuri/ and Daily Post), 

 can scarcely fail to be of considerable interest at once to 

 the geologist and to the coal consumer : — • 



After remarking on the complicated geology of the 

 Bi'istol coal-field, and particularly of the northern part of it, 

 he mentioned that twenty years ago he was able to correct 

 the geological maps of the district by showing that the 

 supposed Millstone Grit, or Farewell Rock, between 

 Bristol and Wick, was one of the Siliceous Sandstones of 

 the coal-measures. It had consequently been discovered 

 that the coal-bearing strata extend south of Kingswood 

 and St. George under the river Avon, and, as far as he 

 knew, to the Mendip Hills. That discovery had had an 

 important bearing on the mining industry of the district, 

 and would help in the future to unlock the mineral re- 

 sources of the neighbouring county of Somerset He had, 

 however, now to describe a discovery he had recently made, 

 which, he believed, would prove of much greater importance. 

 He reminded them that the Kingswood section of the 

 Bristol coal-field contains probably the most ancient coal 

 workings, not only of this country, but probably older 

 than those of South Wales, Somerset, or Dean Forest. In 

 1371 Edward III. issued a mandate to the keeper of the 

 chase of Kingswood to allow Edward, the son of Hugh 

 Blunt, lord of the manor of Bitton, to take, sell, and carry 

 away wood, gorse, and sea-coal found within the demesne ; 

 and by the second half of the seventeenth century he saw 

 by a map which passed to him as lord of the manor, that in 

 the year 1672 there were no less than seventy small coal- 

 pits at work in the Chase of Kingswood. The workings, 

 down to the early part of this century, were of course con- 

 fined to shallow depths, chiefly drained by levels into the 

 Avon and Froom rivers, and were mainly confined to the 

 upper section of the seams now worked in the district. 

 About fifty years ago the Great Vein series were dis- 

 covered, and have been largely worked ever since on the 



