EAR 



EAR 



concealed within it. Before this period 

 little was known of the gravity or densi- 

 ty of the earth, though the relative den- 

 sities of that and others of the planets 

 had been ascertained with tolerable pre- 

 cision. From the data thus acquired, the 

 quantity of matter in the earth is known 

 to be equal to the product of its density 

 by its magnitude ; the force of attraction 

 on the surface has been proved by expe- 

 riments, that substances fall 16 l feet in 

 one second of time ; and hence, that if 

 any place within or beyond it may be 

 known, as in the former case the force is 

 at its distance from the centre, and in the 

 latter as the square of its distance from 

 the same point. 



The earth has three motions : that 

 which gradually occasions the precession 

 of the equinoxes; that round its axis, 

 which causes the succession of day and 

 night, accomplished in 24 hours; and that 

 of the whole mass round the sun in a 

 wide orbit, of which the luminary of day 

 is the centre : in the latter its axis is 

 constantly parallel to itself, and inclined 

 in the same angle to its path ; thus pro- 

 ducing the visible and perceptible altera- 

 tion from spring to summer, and from 

 that period to winter. This annual mo- 

 tion of the earth round the sun is per- 

 formed between the orbits of Venus and 

 Mars, with the former and that of Mer- 

 cury within its own, or between it and 

 the sun in the centre, an<l those of Mars, 

 Jupiter, Saturn, Sic. above it or without, 

 which are called from tin's circumstance 

 superior planets, and the rest inferior. 

 The" time occupied in performing this re- 

 volution is 365 days, 6 hours, and 49 mi- 

 nutes, or a tropical year, calculated from 

 an equinox or solstice to the same again ; 

 the sideral year, as computed from any 

 fixed star to the same point, and seen 

 from the sun, makes the revolving of the 

 earth to occupy 365 days, 6 hours, and 

 9 minutes : the figure of the orbit is el- 

 liptical, with the sun in one focus : the 

 supposed distance is 95 millions of miles, 

 admitting the sun's parallax to be 8.|", or 

 the angle beneath which the semidiame- 

 ter of the earth would appear from the 

 sun; "and the eccentricity of the orbit, 

 or distance of the sun in the focus from 

 the centre of this elliptic orbit, is about 

 i of the mean distance." 



There are nine descriptions of earth, 

 which are, barytes, strontian, lime, mag- 

 nesia, alumina, yttria, glucina, zirconia, 

 and sJica. Every substance is an earth 

 which is insoluble in water, or becomes 



so when combined with carbonic acid, 

 with little taste or scent under the above 

 circumstances, and incapable of altera- 

 tion by fire when in a pure state, then 

 capable of being turned o white pow- 

 der, and not exceeding 4.9 in specific 

 gravity. 



The word earth is generally applied to 

 the whole mass of the globe, and always 

 to the mould which supports the growth 

 of vegetables ; the latter has been atten- 

 tively examined by various chemists, 

 and found to consist of several substances, 

 without any regularity in their arrange- 

 ment. 



EARTHQUAKE. The dreadful conse- 

 quences attending sucden and riolent 

 tremblings of the earih are sufficient 

 reasons for attempting to account for 

 their causes, but, as from the nature of 

 those it is utterly impossible to ascertain 

 them accurately, rational conjecture must 

 be accepted in place of actual observation. 

 Every writer on the effects of volcanic 

 eruptions concurs in attributing earth- 

 quakes to their internal operations, 

 which become less frequent in their 

 neighbourhood after violent explosions 

 through the craters; indeed it must be 

 obvious on the first thought, that the fu- 

 rious efforts of fire, hot air, and steam, 

 exclusive of electric matter, cannot but 

 produce convulsive motion in the sub- 

 stances which confine them, and of more 

 or less extent and violence, in proportion 

 to the cause. When a free passage is ob- 

 tained, the agitation gradually subsides, 

 and is only increased at intervals by the 

 escape of accumulated matter. Sir Wil- 

 liam Hamilton, who carefully examined 

 all the visible phenomena produced by 

 the earthquakes of 1783, in Calabria, is 

 decidedly of opinion, that they originated 

 from under the sea, situated between the 

 coast of Calabria and the island of Strom- 

 boli, or near the city of Oppida, which he 

 illustrates by forming a circle round those 

 points, and remarking that the most de- 

 structive effects were there, whence they 

 became gradually less ruinous. Mount 

 Stromboli had been unusually quiet for 

 some time previous to this calamitous pe- 

 riod, a circumstance highly favourable to 

 his conjecture, founded besides on the 

 experience of remote ages, when, ac- 

 cording to Strabo, Campania was sub- 

 ject to frequent and dreadful earth- 

 quakes, which were less common and 

 ruinous after Vesuvius had become the 

 means of dispersing the confined mat- 

 ter. Since that fortunate aera, the vicini- 

 ty of the above mountain has suffered 



