Page:EB1911 - Volume 20.djvu/816

Rh Atlantic is 54 m. long, and the total length of the river, including the Parabytinga, is 540 m. Its source is about 4920 ft. above sea-level. The Parahyba passes through a fertile, long-settled country, a part of which was for many years the principal coffee-producing region of Brazil. Its lower course passes through the rich alluvial sugar-producing district of Campos. Among the towns on the Parahyba are Campos, São Fidelis, Parahyba do Sul, Juiz de Fora, Barra do Pirahy (railway junction), Rezende, Queluz and Lorena.

PARALDEHYDE, in medicine, a clear colourless liquid (for the chemistry see ), soluble in 1 in 10 of water and freely in alcohol. Paraldehyde is a powerful hypnotic, giving a refreshing quiet sleep which is not followed by unpleasant after effects. As it does not depress the heart when used in medicinal doses, it may be given to patients suffering from cardiac disease. It is much used to produce sleep in the insane. As it is largely excreted by the lungs it may be found useful in bronchial asthma. When taken continuously the drug soon loses its power as a hypnotic. Its unpleasant taste usually prevents the formation of a paraldehyde habit, but it occasionally occurs with symptoms resembling delirium tremens. When taken in an overdose paraldehyde kills by producing respiratory failure.

 PARALLAX (Gr. παραλλάξ, alternately), in astronomy, the apparent change in the direction of a heavenly body when viewed from two different points. Geocentric parallax is the angle between the direction of the body as seen from the surface of the earth and the direction in which it appears from the centre of the earth. Annual parallax is the angle between the direction in which a star appears from the earth and the direction in which it appears from the centre of the sun. For stellar parallaxes see ; the solar parallax is discussed below.

Solar Parallax.&mdash;The problem of the distance of the sun has always been regarded as the fundamental one of celestial measurement. The difficulties in the way of solving it are very great, and up to the present time the best authorities are not agreed as to the result, the effect of half a century of research having been merely to reduce the uncertainty within continually narrower limits. The mutations of opinion on the subject during the last fifty years have been remarkable. Up to about the middle of the 19th century it was supposed that transits of Venus across the disk of the sun afforded the most trustworthy method of making the determination in question; and when Encke in 1824 published his classic discussion of the transits of 1761 and 1769, it was supposed that we must wait until the transits of 1874 and 1882 had been observed and discussed before any further light would be thrown on the subject. The parallax 8.5776″ found by Encke was therefore accepted without question, and was employed in the Nautical Almanac from 1834 to 1869. Doubt was first thrown on the accuracy of this number by an announcement from Hansen in 1862 that the observed parallactic inequality of the moon was irreconcilable with the accepted value of the solar parallax, and indicated the much larger value 8.97″. This result was soon apparently confirmed by several other researches founded both on theory and observation, and so strong did the evidence appear to be that the value 8.95″ was used in the Nautical Almanac from 1870 to 1881. The most remarkable feature of the discussion since 1862 is that the successive examinations of the subject have led to a continually diminishing value, so that at the present time it seems possible that the actual parallax of the sun is almost as near to the old value of Encke as to that which first replaced it. The value of 8.848″, determined by S. Newcomb, was used from 1882 to 1900; and since then the value 8.80″ has been employed, having been adopted at a Paris conference in 1896.

Five fundamentally different methods of determining the distance of the sun have been worked out and appHed. They are as follows:&mdash;

I. That of direct measurement.&mdash;From the measures of the parallax of either Venus or Mars the parallax of the sun can be immediately derived, because the ratios of distances in the solar system are known with the last degree of precision. Transits of Venus and observations of various kinds on Mars are all to be included in this class.

II. The second method is in principle extremely simple, consisting merely in multiplying the observed velocity of light by the time which it takes light to travel from the sun to the earth. The velocity is now well determined; the difficulty is to determine the time of passage.

III. The third method is through the determination of the mass of the earth relative to that of the sun. In astronomical practice the masses of the planets are commonly expressed as fractions of the mass of the sun, the latter being taken as unity. When we know the mass of the earth in gravitational measure, its product by the denominator of the fraction just mentioned gives the mass of the sun in gravitational measure. From this the distance of the sun can be at once determined by a fundamental equation of planetary motion.

IV. The fourth method is through the parallactic inequality in the moon's motion. For the relation of this inequality to the solar parallax see.

V. The fifth method consists in observing the displacement in the direction of the sun, or of one of the nearer planets, due to the motion of the earth round the common centre of gravity of the earth and moon. It requires a precise knowledge of the moon's mass. The uncertainty of this mass impairs the accuracy of the method.

I. To begin with the results of the first method. The transits of Venus observed in 1874 and 1882 might be expected to hold a leading place in the discussion. No purely astronomical enterprise was ever carried out on so large a scale or at so great an expenditure of money and labour as was devoted to the observations of these transits, and for several years before their occurrence the astronomers of every leading nation were busy in discussing methods of observation and working out the multifarious details necessary to their successful application. In the preceding century rehance was placed entirely on the observed moments at which Venus entered upon or left the limb of the sun, but in 1874 it was possible to determine the relative positions of Venus and the sun during the whole course of the transit. Two methods were devised. One was to use a heliometer to measure the distance between the hmbs of Venus and the sun during the whole time that the planet was seen projected on the solar disk, and the other was to take photographs of the sun during the period of the transit and subsequently measure the negatives. The Germans laid the greatest stress on measures with the hehometer; the Americans, English, and French on the photographic method. These four nations sent out well-equipped expeditions to various quarters of the globe, both in 1874 and 1882, to make the required observations; but when the results were discussed they were found to be extremely unsatisfactory. It had been supposed that, with the greatly improved telescopes of modern times, contact observations could be made with much greater precision than in 1761 and 1769, yet, for some reason which it is not easy to explain completely, the modern observations were but little better than the older ones. Discrepancies difficult to account for were found among the estimates of even the best observers. The photographs led to no more definite result than the observations of contacts, except perhaps those taken by the Americans, who had adopted a more complete system than the Europeans; but even these were by no means satisfactory. Nor did the measures made by the Germans with heliometers come out any better. By the American photographs the distances between the centres of Venus and the sun, and the angles between the line adjoining the centres and the meridian, could be separately measured and a separate result for the parallax derived from each. The results were:&mdash;

Transit of 1874:   Distances; par.=8.888″. Pos. angles; par.=8.873″. Transit of 1882:   Distances; par.=8.873″. Pos. angles; par.=8.772″.