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So that, were these islands raised out of the sea, we should have presented to our view a magnificent circular crater about six miles in diameter, bounded by nearly vertical walls of rock from 1000 to 1500 feet in height, and ruptured at one point, from the centre of which would rise two volcanic cones namely, the Kaimenis one with a double crater, still foci of eruption, and from time to time bursting forth in paroxysms of volcanic energy, of which those of 1650, 1707, and 1866 were the most violent and destructive.
The stirring together of the populations that is going on, the fashion and facilities for travel, the production of the radii from the trading foci, are rapidly making a commonplace knowledge of French, German, and Italian a necessity to the merchant and tradesman, and the ever more extensive travelling class.
From the first law of Kepler, namely, the proportionality of the areas to the times of their revolution, Newton inferred that the force which kept the planet in its orbit was always directed to the sun; and from the second law of Kepler, that every planet moves in an ellipse with the sun in one of its foci, he drew the still more general inference that the force by which the planet moves round that focus varies inversely as the square of its distance from the focus.
The moon does not describe a circle round the earth, but rather an ellipse, of which our earth occupies one of the foci; the consequence, therefore, is, that at certain times it approaches nearer to, and at others it recedes farther from, the earth; in astronomical language, it is at one time in apogee, at another in perigee.
"The moon does not describe a circle round the earth, but an ellipse, of which our earth occupies one of the foci; the consequence is, therefore, that at certain times it approaches nearer to, and at others recedes farther from, the earth, or, in astronomical language, it has its apogee and its perigee.
In compound lenses the matter is complicated by the relative foci of its constituents and their distance apart; but these items, in an ordinary photographic objective, would so slightly affect the result that for all practical purposes they may be ignored.
Great efforts were made to develop modern schools, though the work of development was continually hindered by the incessant political unrest. Only at the universities, which became foci of republican and progressive opinion, was any positive achievement possible. Many students and professors were active in politics, organizing demonstrations and strikes.
If microbes are present in the breath of ordinary individuals, what can we expect in the breath of those whose lungs are rotten with tubercular disease? Then we have the collections of expectorated matter and of other organic secretions, which all serve as productive foci.
As the mechanic frequently wants to make an oval having a given width and length, but does not know what the distance between the foci must be to produce this effect, a few directions on this point may be useful: It is a fact well known to mathematicians that if the distance between the foci and the shorter diameter of an ellipse be made the sides of a right angled triangle, its hypothenuse will equal the greater diameter.
A ball of crystal is the commonest and most effectual of such foci, since it has the additional advantage of possessing within itself qualities which stimulate psychic faculty; but other objects are also employed, to which we shall find it necessary to refer more particularly when we come to consider semi-intentional clairvoyance.
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