stitution has increased with a rapidity probably unequaled in any other branch of science. Our knowledge regarding the sun is best arranged under three heads: viz., The general relations of the sun to our globe; the sun's chemical constitution, and its physical constitution. Relations of the Sun to the Earth, as the Source of Light and Heat.-In order to appreciate the grandeur of the scale on which solar activity is carried on, it is only necessary to know a few facts relative to the sun, which are best expressed by numbers.

1. Distance of the Sun from the Earth.-The difficulty in ascertaing the parallax (q.v.) of the sun arises from the smallness of the base line as compared with the distance of the object. The distance of the observing stations must always be less than 8,000 m.; from this the parallax of the moon, which is only 30 times 8,000, can be observed directly with tolerable nearness. But when the distance is many thousands times the length of the base line, the triangle is "ill-conditioned" or unfavorable to accuracy, and the problem must be approached indirectly. The first attempt to measure the distance of the sun was that made by the Greek astronomer Aristarchus in the third c. B. C., who made it only about one-twentieth of what we now know it to be. Even the great astronomer Kepler in the seventeenth c. could only say that the distance must be at least between 13 and 14 millions of miles. Subsequent estimates-for, owing to the imperfection of the methods and instruments, they were little better than estimates-rose to 80 mil. lions. At last, in 1716, the English astronomer Halley proposed a method of employing the transits of Venus. Accordingly, the transits of 1761 and 1769 were observed in a variety of places; but the results at first deduced were discordant and unsatisfactory, until in 1824 the German astronomer Encke "discussed" the observations of 1769, and arrived at a distance of about 95 millions of miles; and this number held its place in books of astronomy for a good many years. In the mean time, in the absence of transits, other methods, become possible through the growing perfection of astronomical instruments, were tried, and most of them concurred in pointing to a value nearly 34 millions less than that above stated; so that 91,500,000 came to be accepted as the approximate distance of the sun, until the transit of 1874 should settle it more definitely.

A transit can occur only when the planet is in or near one of her nodes at the time of inferior conjunction, so as to be in a line between the earth and the sun. The coincidence of these two conditions follows a rather complex law. There are usually two transits within eight years of one another, and then a lapse of 105 or 122 years, when another couple of transits occur, with eight years between them. The transit of 1874 was succeeded by that of 1882, and there will not be another until June, 2,004.

The way in which a transit is turned to account may be understood by the help of the accompanying diagram, where E represents the earth; V, Venus; and S the sun. It is to be premised that the relative distances of the planets from the sun are well known. Their periodic times can be observed with accuracy, and from these by Kepler's (q.v.) law we can deduce the proportions of the distances, but not the distances themselves. It is thus known that if the distance of the earth from the sun is taken as 100, that of Venus is 72. In the fig. then, AV is 28, or about one-third of Va or Vb.

An observer at a station, A, on the northern part of the earth will see the planet projected on the sun as at a, while a southern observer will see it at b. The distance of the sun from Venus being about three times her distance from the earth, it is obvious that

But how is it measured? For each observer sees only one of the spots, and does not know where the other is; and there are no permanent marks on the sun's surface to guide us. The difficulty is got over in the following way: Each observer notes the exact duration of the transit, that is, the time the spot takes to travel from C to D, or from F to G. Now as we know the rate of Venus's motion in her orbit, this gives us the lengths of the lines CD and FG in minutes and seconds of arc. Knowing then the angular diameter of the sun (32') and the lengths of two chords CD and FG, we can easily, by the properties of the circle, find the distance ab between them. This gives us the angle a Ab. In the triangle AVb, then, we know the angle at A and the proportion of the sides AV and Vb, and from that we can find the angle AbV or AbB. Now this is the quantity sought, being the parallax of the sun as seen from two stations on the earth. Whatever the distance AB actually is, the angle is reduced to correspond to a distance equal to the earth's semi-diameter. The parallax deducted by Encke, as above referred to, was only 8.5776", while the parallax corresponding to the other smaller measurement above stated is 8.94". The advantage of this roundabout procedure is that a comparatively large angle (aAb) is measured in order to deduce from it a smaller (AbB), so that any error in the measurement is diminished in the result.

The transit of 1874 was observed at more than fifty stations, astronomers from all the civilized world taking part in the work. The labor of discussing and comparing the observations has not yet been overtaken, but several partial results have been announced,

which still show considerable discrepancy. The chief source of uncertainty arises from the difficulty the observers found in determining the exact moment of ingress" and "egress" of the planet, owing to the dense atmosphere of the latter rendering the limbs of the two bodies indistinct and distorted. Much was expected from the multitude of photographic pictures taken, but they have proved a failure. They are said to lack the necessary sharpness, ard to be liable to other sources of error. The first partial discussion of the British observations gave, according to the astronomer royal, a result of 93 millions of miles. A more extended discussion since announced results in 92 millions of miles.

Elaborate arrangements were made, on an international plan, for observing the transit of 1882, and attempts made to obviate the defects of previous observations. Previous to this astronomers were turning with greater hope to other methods, especially to observations of Mars, and of some of the minor planets. From observations of Mars made in 1862, the American astronomer Newcomb deduced a distance of 92 millions of miles. The velocity of light, which has been determined by the ingenious optical experiments of Foucault and others, has also been pressed into the service of the problem. The aberration of light (q.v.) results from the relation of the velocity of light to that of the earth's motion in her orbit; and from the observed amount of the aberration we are thus able to deduce the earth's velocity. From knowing then the time of the earth's revolution, we can find the circumference of her orbit, and hence her distance from the sun. The most careful investigation by this method gives a distance of 93 millions of miles. An ingenious method of observing the parallax of Mars at its opposition, first suggested by the astronomer royal, but carried out by Mr. Gill on the island of Ascension in 1877, promises still more satisfactory results. The essence of the method consists in this, that instead of depending upon two sets of observers at different parts of the earth, one observer and one station are made to suffice. One observation is taken in the evening when the planet is rising, and another in the early morning when it is setting. In the mean time the rotation of the earth has transported the observer 6,000 or 7,000 m. through space, and this forms his base line. Mr. Gill's observations were made by means of the heliometer, the most effective of instruments for such purposes. From such of his observations as had been reduced at the end of 1878, Mr. Gill announces his belief that the sun's distance will prove to be nearer to 93 than to 92 millions of miles.

The other important numerical facts relative to the sun are the following: Its diame ter calculated on the basis of the shorter distance hitherto received, is, in round numbers, 850,000 m., or more than 107 times the mean diameter of the earth; so that the volume or bulk of the sun exceeds that of the earth 1,200,000 times, and is 600 times greater than the bulk of all planets at present known, together. The mass of the sun, or quantity of matter it contains as measured by weight, exceeds that of the earth only 300,000 times; and thus it appears that the matter of the sun has only one-fourth the density of that of the earth. From this and other facts, it is inferred that the matter of the sun exists for the most part in a gaseous condition. Still his mass is 740 times greater than the masses of all known planets put together. The period of rotation of the sun upon its axis, which Galileo was the first to calculate from observations of the sunspots, and which takes place in the same direction as that of the earth, is about 25 days 8 hours. It appears, however, that this period varies according to the solar latitude of the spots from which it is calculated. The inclination of the axis of the sun to the ecliptic is about 74, and the longitude of the ascending node is about 74° 30'.

2. The form or figure of the sun has been the subject of recent investigations. The polar and equatorial diameters of the sun's disk as observed, have been supposed to differ, though by a very small quantity only. The photographs of the sun do not quite agree in the amount of the value for the diameter with that given by observations.

The general laws by which the relation of our earth to the sun, as the source of light and heat, is governed, are of the most simple kind. The rays which emanate from the sun's disk into space proceed in diverging lines, and, on arriving at the earth, their intensity will be inversely proportional to the square of the sun's distance. This may be called the primary law; but the more obvious phenomena of solar heat and light are manifested to us under a secondary law depending on the obliquity of incidence of the sun's rays. See CLIMATE; EARTH; HEAT; LIGHT, etc.

3. Chemical Constitution of the Sun.-Astronomy has weighed and measured the sun long ago, and in our days chemistry, aided by physics, makes an analysis of it. The way in which this surprising result is arrived at is explained under SPECTRUM. The main fact on which the method rests is briefly this: that a substance, when comparatively cold, absorbs the very same rays which it gives out when heated. Hence it was inferred by Kirchhoff that if there was sodium or iron in a comparatively cold state in the solar atmosphere, above the source of light, these substances would produce black lines corresponding in spectral position with the bright lines which they give out when heated. On this principle the presence in the solar spectrum of hydrogen, magnesium, calcium, sodium, and metals of the iron group has been ascertained with something like certainty. There are less clear indications of other metals, such as zinc and lead; while metals of the tungsten, antimony, silver, and gold classes have been searched for in vain. Of the metalloids, such as oxygen, carbon, nitrogen, sulphur, and the like, none had been detected till, in 1877, Prof. Henry Draper of America announced the discovery