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taining the dimensions of the solar system, and provides an invariable foundation for a system of weights and measures. The mutual attraction of the celestial bodies disturbs the fluids at their surfaces, whence the theory of the tides and of the oscillations of the atmosphere. The density and elasticity of the air, varying with every alternation of temperature, lead to the consideration of barometrical changes, the measurement of heights, and capillary attraction; and the doctrine of sound, including the theory of music, is to be referred to the small undulations of the aërial medium. A knowledge of the action of matter upon light is requisite for tracing the curved path of its rays through the atmosphere, by which the true places of distant objects are determined, whether in the heavens or on the earth. By this we learn the nature and properties of the sunbeam, the mode of its propagation through the ethereal medium, or in the interior of material bodies, and the origin of colour. By the eclipses of Jupiter's satellites the velocity of light is ascertained ; and that velocity, in the aberration of the fixed stars, furnishes a direct proof of the real motion of the earth (N. 237). The effects of the invisible rays of the spectrum are immediately connected with chemical action ; and heat, forming a part of the solar ray, so essential to animated and inanimated existence, is too important an agent in the economy of creation not to hold a principal place in the connexion of physical sciences; whence follows its distribution in the interior and over the surface of the globe, its power on the geological convulsions of our planet, its influence on the atmosphere and on climate, and its effects on vegetable and animal life, evinced in the localities of organized beings on the earth, in the waters, and in the air. The correlation between molecular and chemical action, light, heat, electricity, and magnetism, is continually becoming more perfect, and there is every reason to believe that these different modes of force, as well as gravity itself, will ultimately be found to merge in one great and universal power. Many more instances might be given in illustration of the immediate connexion of the physical sciences, most of which are united still more closely by the common bond of analysis, which is daily extending its empire, and will ultimately embrace almost every subject in nature in its formulæ.
These formulæ, emblematic of Omniscience, condense into a
few symbols the immutable laws of the universe. This mighty instrument of human power itself originates in the primitive constitution of the human mind, and rests upon a few fundamental axioms, which have eternally existed in Him who implanted them in the breast of man when He created him after His own image.
NO TE S.
Note 1, page 2. Diameter. A straight line passing through the centre, and terminated both ways by the sides or surface of a figure, such as of a circle or sphere. In fig. 1, q Q, NS, are diameters.
NOTE 2, p. 2. Mathematical and mechanical sciences. Mathematics teach the laws of number and quantity ; mechanics treat of the equilibrium and motion of bodies.
NOTE 3, p. 2. Analysis is a series of reasoning conducted by signs or symbols of the quantities whose relations form the subject of inquiry.
NOTE 4, p. 3. Oscillations are movements to and fro, like the swinging of the pendulum of a clock, or waves in water. The tides are oscillations of the sea.
NOTE 5, p. 3. Gravitation. Gravity is the reciprocal attraction of matter on matter ; gravitation is the difference between gravity and the centrifugal force induced by the velocity of rotation or revolution. Sensible gravity, or weight, is a particular instance of gravitation. It is the force which causes substances to fall to the surface of the earth, and which retains the celestial bodies in their orbits. Its intensity increases as the squares of the distance decrease.
NOTE 6, p. 4. Particles of matter are the indefinitely small or ultimate atoms into which matter is believed to be divisible. Their form is unknown; but, though too small to be visible, they must have magnitude.
NOTE 7, p. 4. A hollow sphere. A hollow ball, like a lomb-shell. A sphere is a ball or solid body, such, that all lines drawn from its centre to its surface are equal. They are called radii, and every line passing through the centre and terminated both ways by the surface is diameter, which is consequently equal to twice the radius. In fig. 3, Qq or NS is a diameter, and CQ,
Fig. 1. CN are radii. A great circle of the sphere has the same centre with the sphere as the circles Q E qd and Q N q S. The circle A B is a lesser circle of the sphere.
NOTE 8, p. 4. Concentric hollow spheres. Shells, or hollow spheres, having the same centre, like the coats of an onion.
NOTE 9, p. 4. Spheroid. A solid body, which sometimes has the shape of an orange, as in fig. 1 ;
it is then called an oblate spheroid, because it is fattened at the poles N and S. Such is the form of the earth and planets. When, on the con
trary, it is drawn out at the poles like an Fig. 2.
egg, as in fig. 2, it is called a prolate spheroid. It is evident that in both these solids the radii Cq, Ca, CN, &c., are generally unequal ; whereas in the sphere they are all equal.
NOTE 10, p. 4. Centre of gravity. A point in every body, which if supported, Q the body will remain at rest in whatever
position it may be placed. About that point all the parts exactly balance one another. The celestial bodies attract each other as if each were condensed into a single particle situate in the centre of gravity, or the particle situate in the centre of gravity of each
may be regarded as possessing the resultant power of the innumerable oblique forces which constitute the whole attraction of the body.
NOTE 11, pp. 4, 6. Poles and equator. Let fig. 1 or 3 represent the earth, C its centre, NCS the axis of rotation, or the imaginary line about which it performs its daily revolution. Then N and S are the north and south poles, and the great circle q E Q, which divides the earth into two equal parts, is the equator. The earth is flattened at the poles, fig. 1, Fig. 3.
the equatorial diameter, 9 Q, exceeding the polar diameter, NS, by about 261 miles. Lesser circles, A BG, which are parallel to the equator, are circles or parallels of latitude, which is estimated in degrees, minutes, and seconds, north and south of the equator, every place in the same parallel having the same latitude. Greenwich is in the parallel of 51° 28' 40". Thus terrestrial latitude is the angular distance between the direction of a plumb-line at any place and the plane of the equator.
Lines such as NQS, NGES, fig. 3, are called meridians; all the places in any one of these lines have noon at the same instant. The meridian of Greenwich has been chosen by the British as the origin of terrestrial longitude, which is estimated in degrees, minutes, and seconds, east and west of that line. If NGES be the meridian of Greenwich, the position of any place, B, is determined, when its latitude, QC B, and its longitude, ECQ, are known.
NOTE 12, p. 4. Mean quantities are such as are intermediate between others that are greater and less. The mean of any number of unequal quantities is equal to their sum divided by their number. For instance, the mean between two unequal quantities is equal to half their sum.
NOTE 13, p. 4. A certain mean latitude. The attraction of a sphere on an external body is the same as if its mass were collected into one heavy particle in its centre of gravity, and the intensity of its attraction diminishes as the square of its distance from the external body increases. But the attraction of a spheroid, fig. 1, on an external body at m in the plane of its equator, E Q, is greater, and its attraction on the same body when at m' in the axis N S less, than if it were a sphere. Therefore, in both cases, the force deviates from the exact law of gravity. This deviation arises from the protuberant matter at the equator; and, as it diminishes towards the poles, so does the attractive force of the spheroid. But there is one mean latitude, where the attraction of a spheroid is the same as if it were a sphere. It is a part of the spheroid intermediate between the equator and the pole. In that latitude the square of the sine is equal to 1 of the equatorial radius.
NOTE 14, p. 4. Mean distance. The mean distance of a planet from the centre of the sun, or of a satellite from the centre of its planet, is equal to half the sum of its greatest and least distances, and, consequently, is equal to half the major axis of its orbit. For example, let PQ AD, fig. 6, be the orbit or path of the moon or of a planet; then P A is the major axis, C the centre, and CS is equal to CF. Now, since the earth or the sun is supposed to be in the point S according as PDAQ is regarded as the orbit of the moon or that of a planet, SA, SP are the greatest and least distances. But half the sum of S A and S P is equal to half of A P, the major axis of the orbit. When the body is at Q or D, it is at its mean distance from S, for SQ, SD, are each equal to C P, half the major axis by the nature of the curve.
NOTE 15, p. 4. Mean radius of the earth. The distance from the centre to the surface of the earth, regarded as a sphere. It is intermediate between the distances of the centre of the earth from the pole and from the equator.
NOTE 16, p. 5. Ratio. The relation which one quantity bears to another.
Fig. 4. NOTE 17, p. 5. Square of moon's distance. In order to avoid large numbers, the mean radius of the earth is taken for unity: then the mean distance of the moon is expressed by 60; and the square of that number is 3600, or 60 times 60. NOTE 18, p. 5. Centrifugal force. V K
Sr q P The force with which a revolving body tends to fly from the centre of motion: a sling tends to fly from the hand in consequence of the centrifugal force. A tangent is a straight line touching a curved line in one point without cutting it, as m T, fig. 4. The direction of the centrifugal force is in the tangent to the curved line or path in which the body revolves, and its intensity increases with the angular swing of the body, and with its distance from the