m 



EARTH. MKAN PKNSITY OF THK. 



EARTHENWARE. 



714 



experiment*, by the usual rules of the theory of probabUities it is an 

 ereo chance that the error of this remit is within -04. Cavendish's 

 result was 5-48. 



Besides the confirmation of some of the most material potato of the 

 theory of gravitation which results from this experiment, it furnishes 

 a prwumption of UM strongest kind that the earth is solid to the 

 centre, and not, as many have suppomd in every age, a hollow shell. 

 The mean density 51 k Ten much greater than that of the substances 

 which abound at the surface. All common rocks are under 3, and 

 nothing under the ores of the heavier metals comes up to 5]. The 

 earth is a> tn ' '-* as if it were all composed of silver-ore, from the 

 centre to the circumference, so that there must be an increase of 

 density towards the centre. If those who think the earth to be a shell 

 were to presume that it* solidity ceased at five hundred miles below 

 UM surface, they would then be compelled to give to the terrestrial 

 mtW-r. one part with another, a density greater than that of mercury, 

 in order that the whole shell, the hollow part included, might have 

 the mean density which is found by this experiment. 



Recently, Hr. Airy, the present Astronomer Royal, has determined 

 the mean density of the earth by means of observations of the rate of 

 oscillation of a pendulum made at the top and bottom of a coal-mine 

 in the county of Durham. As early as the year 1826 he endeavoured, 

 in conjunction with Dr. W he well and the late Hr. Sheepshanks, to effect 

 the same object by means of pendulum experiments in the Dolcoath 

 mine in Cornwall, but the project was frustrated by local accidents ; 

 and a similar attempt in 1 828 proved equally fruitless. The prin- 

 cipal obstacle to the success of the experiments on those occasions 

 consisted in the difficulty of comparing the clocks at the top and 

 bottom of the mine. In more recent times the application of galvanic 

 electricity to the transmission of signals suggested to Hr. Airy an 

 easy method of conquering this impediment. He accordingly re- 

 solved to repeat the experiment, and chose for that purpose the Harton 

 colliery, near South Shields, the bottom of which is no less than 1260 

 feet beneath the earth's surface. The application of pendulum 

 experiments made at the earth's surface, and at some distance beneath 

 it, to the determination of the mean density of the earth, rests on two 

 prim-ipKw of attraction, both of which were originally established by 

 Newton. The first U, that the attraction exercised upon an external 

 ]>article by a sphere of uniform density, or one consisting of concentric 

 strata of different densities, but of uniform density throughout each 

 stratum, is the same as if all the matter of the sphere were collected 

 at the centre. The second is, that a hollow spherical shell of matter 

 exercises no effective attraction on a particle placed anywhere within 

 it. Now, in the case of experiments made with a pendulum at the 

 top and bottom of a coal-mine, the pendulum is acted upon in the 

 funner instance by a force equivalent to the attraction which the 

 whole quantity of matter contained in the earth would exert if it were 

 collected at the centre ; in the latter instance the quantity of attract- 

 ing matter is leas than in the former by the spherical shell, whose 

 thickness is equal to the distance between the top and bottom of the 

 mine, but the distance of the particle from the centre of attraction is 

 less. It is clear then, that experiments founded on the principles 

 to which we have just alluded, enable us to weigh the whole mass 

 of the earth against an exterior shell of a given thickness ; and there- 

 fore if the density of the shell (or at any rate the density of the 

 parts which are nearest the mouth of the mine, and on which the 

 attraction exerted by the shell upon the pendulum mainly depends), 

 be ascertained by observation, we shall hence readily deduce the value 

 of the mean density of the earth. Hr. Airy's experiments on the rate 

 of oscillation of the pendulum at the top and bottom of the Harton 

 colliery were made in the summer of 1854. Several English observa- 

 tories co-operated in their execution by furnishing assistant observers. 

 The Electric Telegraph Company supplied the means of establishing 

 simultaneous galvanic signals between the upper and lower stations. 

 The observations consisted of 104 hours of incessant observations of 

 one pendulum A, above, and another pendulum B, below ; then of 

 104 noun with B above, and A below ; then of 60 hours with A above 

 and B below : then of 60 with B above and A below. It appeared from 

 these experiments that the lower pendulum was accelerated 2'25" per 

 day, or in other words, that the force of gravity was more intense at 

 the lower station than at the upper by j^-th part. The final con- 

 clusion which Hr. Airy deduced from these experiments was, that the 

 mean density of the earth is 6-623, the mean density of water being 

 represented by unity. This result, it will be seen, is considerably 

 higher than any value of the same element hitherto found. 



The most recent operations for determining the mean density of the 

 earth are due to Colonel James, Superintendent of the Ordnance 

 Survey, who, in 1855, caused a series of celestial observations to be 

 made to the north and south of Arthur's Seat, near Edinburgh, for 

 the purpose of ascertaining the amount of deflection caused by the 

 attraction of the mountain. The resulting value of the mean density 

 was found to be 5'316, with a probable error of 0-054. Colonel James 

 purposes, as soon as the details of the survey of Ben Nevis have been 

 completed, to obtain a new solution of the important problem of the 

 mean density of the earth by determining the amount Of deflection 

 caused by the attraction of that mountain, which is the highest in the 

 Brituh Isles, and appears to be in other respects well adapted for the 

 object in view. 



The reader may consult the following recent works on this subject : 

 ' ExperimpiiU with the Torsion-rod, for determining the mean density 

 of the Earth,' by Francis Baily, Esq. (' Hem. Ast. Soc.,' vol. xiv.f; 

 'Account of Pendulum Experiments undertaken in the Horton <'! 

 liery, for the purpose of determining the mean density of the Earth ' 

 (with a Supplement), by G. B. Airy, Esq. (' Phil. Trans.' 1856, vol. 

 cxlvi.); 'On the deflection of the plumb-line at Arthur's Seat, and 

 the mean specific gravity of the Earth/ by Capt, Clarke/ R.E. (' Phil. 

 Trans.' 1856, voL cxlvi.) 



EARTHENWARE. The art of moulding earthen vessels for do- 

 mestic use appears to have been practised in the earliest ages, and 

 among the rudest nations. It is at once the most ancient and the 

 most widely diffused of the arts. In newly-discovered countries it has 

 been found that the use of earthen vessels is fmiljr among people 

 otherwise little acquainted with the arts of civilised life. Vases 

 have been discovered in the ruins of temples and palatial buildings, 

 constructed by the Aztecs, and other aboriginal tribes of central 

 America ; and there is strong evidence for believing that these vessels 

 were the manufacture of the country in which they were found. 

 The potter's wheel is represented in the sculpture and punting of 

 ancient Egypt ; frequent reference is made to it in the scriptures of 

 the Old Testament ; it must have been in use from a very remote 

 date in ancient Assyria, China, and Japan. In Greece, Rome, and 

 Etruria the potter's art wax cultivated with the greatest diligence from 

 the earliest and rudest period of their respective histories, to that of 

 their highest prosperity and refinement. The art indeed arrived at a 

 point of great advancement among the oldest of these nations. In 

 ancient Egypt were made vases and other articles of very elegant 

 forms, as well as common ware for domestic use. From the analyses 

 mode at the laboratory of the Museum of Practical Geology, it appears 

 that the pastes or bodies used in making Egyptian earthenware, as seems 

 indeed to have been the case with most ancient pottery, were merely 

 " the natural clays selected for their fitness to the purposes for which 

 they were intended." Glazes or enamels were used by the Egyptians 

 for covering small figures made of a frit of sand, but it is believed that 

 it was not till the later periods of their history that they acquired the 

 art of applying vitreous glazes to their earthenware proper. Tho 

 Assyrian and Babylonian pottery, as Hr. Birch remarks in his ' History 

 of Ancient Pottery,' "although it bears a general resemblance in 

 shape, form, and use, to that of Egypt, has certain specific differences. 

 As a general rule it may be stated to be firmer in its paste, brighter in 

 its colour, employed in thinner masses, and for purposes not known in 

 Egypt" The Assyrians were, from a very early period, fully acquainted 

 with the use of siliceous and metallic glazes or enamels, and produced 

 them of brilliant colours. Very little of their earthenware has, how- 

 ever, been recovered. 



In the hands of the Greeks, vessels of earthenware assumed forms 

 of the most perfect grace and symmetry ; and much additional interest 

 was imparted to them for future ages by the practice of painting on 

 them designs which serve now as the truest reflex of Greek pointing of 

 the several periods, and afford an almost limitless store of illustration 

 of Greek customs, mythology, Ac. The earliest remaining examples 

 of Greek fictile vases are of a pale yellow clay, and have the designs 

 rudely painted of a dark reddish brown. Later, the figures were 

 painted in a black glaze on a pale or red ground. But in the best 

 period of Greek art the earthenware was formed of a reddish substance, 

 which was glazed black ; the figures of the design being left of the 

 natural red of the ground. The shapes ore of almost infinite variety, 

 but almost invariably marked by the most beautiful simplicity of out- 

 line. The pottery of Athens was the most celebrated in Greece, and 

 Athenian vases were offered as prizes in the public gomes. That of 

 Samoa was famous in the days of Homer. With the decline of Greece, 

 all its arts gradually declined, and eventually perishnd together. 

 Ancient Greek vases ore among the most esteemed objects in museums 

 of antiquities : during the lost century in particular, extraordinary 

 prices were given for fine specimens : a famous vase, containing a 

 representation of the last night of Troy, was purchased for the Huseo 

 Borbonico for 15002. But owing to the vast numbers of them, and of 

 Etruscan vases, which ore essentially similar in character, which l,.u.- 

 been found, the prices have fallen considerably, though very high 

 prices are still readily given for superior examples. [VASES.] 



Roman pottery is, in on artistic point of view, of far less value than 

 that of Greece, but as a manufacture much of it is of a very superior 

 quality. The use of earthenware for domestic purposes was almost 

 universal with the Roman people, and wherever they settled tln-v 

 appear to have carried the manufacture with them ; not only Roman 

 pottery, but traces of kilns for the firing of the ware having been 

 found in most of the countries in which were Roman settlements. 

 The ware itself is usually of a bright red colour, given to it by the 

 introduction of a peroxide of iron into the paste, and it bears a 

 brilliant glaze. H. Brongniart calls particular attention to the per- 

 fection of workmanship exhibited in this ware, in the malting of which 

 he says most of the processes now in use appear to have been 

 employed. 



With the fall of the Roman empire the arts for a time perished also. 



Communicated by Colonel James, under whose Bupcrlntcndcnco tbe whole 

 operation wu conducted. 



