LESSONS IN GREEK. 



tiv.-ts of tin., ..,iiti!iul wear are very great, 

 a i.u-K'" uinounL i.t" Mlt in.. I a,ud being carried down and do- 



i at the bottom of lakes or of the aea. 



.t niii.il I.IOCOMH of ruducing the height* and filling up the 

 hollows on tli.- !.i<-.> ..c tho earth uthua very slowly being carried 

 on, uiul to the name caiue may probably be attributed many of 

 the great geological changed which the surfaoe of the earth ban 

 untamed in former ago*. Nearly all the solid rook* of the earth 



11 fact, deposited under water, and are composed chiefly of 



i hiiH worn down from surrounding parts by the action of 



If we just consider tho amount of rain that fall* in any place 

 we shall see what an immense power is stored up in it, which ia 

 partly exerted in thus wearing down the surface. A largo 

 amount of power in, however, wasted. In fact, some largo 

 . s and waterfalls have in them almost inexhaustible stores 

 of power, but little of which ia turned to account. 



It ia calculated that tho total animal rainfall in England ia 

 about two feet, that ia, it would cover the aurfaco uniformly 

 to that depth. Now since tho weight of a cubic foot of water 

 ia about 62J pounds, we aee that the weight of rain falling on 

 every aquaro foot of surface ia upwards of a hundred- weight; 

 the weight per acre ia therefore about 2,400 tona. The mean 

 elevation of England may probably bo taken at upwards of 300 

 feet. Tho power, therefore, thus stored up in the rain in 

 more than 2,400 x 300 tons per acre. If, therefore, the fears 

 about the exhaustion of coal ever should be realised, we shall 

 find here a copioua supply of moving force, and doubtless 

 machines would soon be invented to render a much larger por- 

 tion of it available. 



In tropical regions the rainfall is very much greater than in 

 tho latitude of England, in some places reaching as much as 19 

 or 20 feet in the year. The mean depth over the whole sur- 

 face of the earth may, perhaps, be set down at about five feet. 

 Tho mean elevation, too, of the rest of the world is very much 

 greater than that of England. Wo see, then, what an immense 

 amount of power is thus produced ; and as it is the heat of the 

 sun which turns tho water into vapour, and thus raises it, to fall 

 again in the shape of rain, the sun may be said to bo the source 

 of all this power. 



Wo have next to consider the motions of the sea, the most 

 important and regular of which are tho tides. In every part of 

 the sea, or of a large river, the height of the water is found to 

 vary from hour to hour, attaining its maximum twice in the 

 twenty-four hours, and at intermediate periods being at its 

 lowest level. These alternations always attract attention by their 

 regularity, and it is found that the period of high water is about 

 fifty minutes later on any given day than it was on the preced- 

 ing day. These motions arise from the attraction of the moon, 

 and will easily be understood by reference to the annexed 

 figure. 



M 





Fig. 42. 



Let E represent the earth, and M the position of the moon. 

 Like all other bodies, they attract one another, but since the 

 earth is solid its shape is not at all affected by this attraction. 

 The water, however, is movable, and therefore flows from those 

 parts which are away from the direct influence of the moon to- 

 wards those which are vertically under it, and thus causes high 

 tide at the latter. Now as tho earth revolves on its axis in 

 twenty-four hours, this would cause high water at each part of 

 the earth's surface once in the day, but, as we have seen, there 

 are two tides in the day. How, then, is the second to be ac- 

 counted for. Partly by the fact that as the moon attracts tho 

 water on the side nearer to it more powerfully than it does the 

 earth, so it attracts the earth moro than the water upon the 

 further side, which is therefore, as it were, left behind. This, 

 however, will not fully account for the two tides; and the other 

 cause is found in the fact that the moon does not revolve round 

 the centre of the earth, but round the centre of gravity of tho 

 earth and itself. This point is situated on the line E M, and 



therefor* the water at y, being MOM dirtant from it than that 

 at , i* heaped op by eeothf ogai force, and thu* aid* in pro. 



du-ii,g th., ..,.:'.!..! U !.-. 



The reaaon why the tide* are aboot fifty minate* later every 

 day U found in the fact that the BOOB ha. a motion of >U own. 

 by which it revolvee round the earth io about a month; aad the* 

 when after the lapae of twenty-four hoar* the part of the 

 earth' nurfaoe which was vertically under the moon ocmee Into 

 the same place again, the moon baa Moved onward*, aad the 

 earth must therefore rotate a little further before the moon i* 

 gain vertical to that place. 



In the open aea the height to which the tidal wave ri*ee i* but 

 mall, but in aome plaoee it bae to paa along a Barrow channel. 

 and then rise* to an uuiuual extent Thi* u well eeen ia the 

 Day of Fundy, where the wave U driven along a narrowing 

 channel, and rises upward* of aizty feet. A similar thing meer* 

 in the British Channel. In the Thanes, a little below London 

 Bridge, the riae of the tide ie about nineteen feet. The rate at 

 which the tidal wave travels varies oonaiderably with the depth ; 

 in the open sea its rate u about 1,000 milee per boor, bat 

 when its channel is narrower and more ahallow the apeed ie very 

 greatly diminished. Sometimes, too, tidal wavee which have 

 travelled in different direction* meet, and thus produce apparent 

 irregularities. 



The other motion of the aea is that of its wavee. If we watch 

 its surface we observe many small elevations apparently moving 

 onwards. These sometimes acquire a considerable height in the 

 open sea, so that a vessel ia very much toeeed about by their 

 action ; the height to which they rise is, however, usually very 

 much over-estimated. Noir if we watch closely, and obeerve 

 pieces of weeds or sticks floating on the surface, or even the 

 bubbles, we shall Boon see that the particles of water do not 

 themaelvea move onwards with the wave. They alternately riae 

 and fall, and also have a alight motion to and fro, bat they re- 

 main almost in the same position, except when the wave daahee 

 upon the shore; just the same as on a windy day a corn-field 

 presents the appearance of wavee travelling acroes it, when we 

 well know that it is merely the waving of each of the ears of the 

 corn in succession. 



The whole subject is, however, too complicated to be fully 

 examined here, and as there are other branches of Natural ttiilo- 

 sophy which claim our attention, wo must leave it now, and 

 thus conclude our sketch of Hydrostatics. 



LESSONS IN GREEK. XXXIX. 



PABADIGMS OF MUTE VEBBS. 



1. VERBS WHOSE CHARACTERISTIC is A p SOUND (w, 0, f ). 

 (a) Pure Characteristic w, 0, <p ; fut. if*. 



ACTIVE VOICE. 



Pres. ind. rpl^-v, I nib, snbj. rplfat, imp. rpift-i, inf. - 

 part. rplfr-tav. 



Impf. ind. t-rpifl-ov, opt. rplff-otfu. 

 Fut. ind. (rptft-ffo>) rpi^ta, opt. rpnj/oifju, inf. Tpnfrfur. 

 Aor. 1 ind. f-rpiifo, subj. rpiifn, opt. rpi^cuju, imp. 

 nf. rpiij/ai, part, rpiif ai. 



Perf. 1 ind. (rt-rpl^-a) rc-rpif a, *ubj. rt-Tpi++t, imp. T- 

 rpi$>-, inf. Tt-Tpi<p-tvai, part. Tt-rpi$-mt. 

 Plup. 1 ind. ((--rt-Tpi&-tu>) t-Tt-rplf-tu'. 



MIDDLE VOICE. 



Pres. ind. rplftofiat. Impf. ind. -rpi#-w. 



Fut. ind. Tpityopcu. Aor. 1 ind. (rpi^o^nr. 



Perf. ind. Tf-rpi-muu, -^cu, -TTCU, etc. ; imp. rt~rpi-^, 

 etc. ; inf. Tf-rpi-<p6eu ; part. rf-Tpip-pcyot ; subj. 

 -o> ; opt. rt-Tpin-fi.(voi, -fujy. 



Plup. t-rt-T pip-Mr, -ifio, -TO, etc. Fut 3 ind. 

 PASSIVE VOICE. 



Aor. 1 ind. (t--rpi&-to\i>) t-Tpt<j>-6rn>. Fnt. 1 ind. 



Aor. 2 ind. e-rplg-jjf Fut. 2 ind. 



Verbal Adj. (rpt^-ros) rptw-ros, -if, -o* ; rptw-rtot, -, *. 



(6) Impure Characteristic r ; fut. jut. 



Active. Middle. POUVM. 



Pre*. KOWT*, J knock i KOKT^MU. 



Perf. 1. K-<c<xJ>a, Mwyyuu, like rrrptftf^ 



Perf. 2. Kficova (Horn.), Aor. 1. 



