“At the vernal equinox the appearance of the zodiacal light is that of a pretty broad pyramidal, or rather lenticular, body of light, which begins to be visible as soon as the twilight decays. It is very bright at its lower or broader part near the horizon, and, if there be broken clouds about, often appears like the glow of a distant conflagration, or of the rising moon, only less red, giving rise, in short, to amorphous masses of light such as have been noticed by one of your correspondents as possibly appertaining to the comet. At higher altitudes, its light fades gradually, and is seldom traceable much beyond the Pleiades, which it usually, however, attains and involves, and (what is most to my present purpose) its axis at the vernal equinox is always inclined (to the northward of the equator) at an angle of between 60° and 70° to the horizon, and it is most luminous at its base, resting on the horizon, where also it is broadest, occupying, in fact, an angular breadth of somewhere about 10° or 12° in ordinary clear weather.”
[17] “The assumption that the extent of the starry firmament is literally infinite has been made by one of the greatest of astronomers, the late Dr. Olbers, the basis of a conclusion that the celestial spaces are, in some slight degree, deficient in transparency; so that all beyond a certain distance is, and must remain for ever, unseen; the geometrical progression of the extinction of light far outrunning the effect of any conceivable increase in the power of our telescopes. Were it not so, it is argued, every part of the celestial concave ought to shine with the brightness of the solar disc, since no visual ray could be so directed as not, in some point or other of its infinite length, to encounter such a disc.”—Edinburgh Review, p. 185, for January, 1848; Etudes d’Astronomie Stellaire.
[18] In the Astronomische Nachrichten of July, 1846, appeared a Memoir by M. Mädler, Die Centralsonne. The conclusions arrived at by Mädler may be understood from the following quotation from a French translation, made by M. A. Gautier, in the Archives des Sciences Physiques et Naturelles, for October, 1846:—“Quoiqu’il résulte de ce qui précède que la région du ciel que j’ai adoptée satisfait à toutes les conditions posées plus haut, il n’en est pas moins convenable de la soumettre à toutes les épreuves possibles. Plusieurs essais de combinaisons différentes m’ont convaincu qu’on ne pourrait trouver aucun autre point dans le ciel qui pût tenir lieu, même d’une manière approchée, que celui que j’ai adopté. On pourrait maintenant m’addresser l’objection que, si la région du ciel où se trouve le centre de gravité de notre système d’étoiles fixes, est déterminée par ce qui précède entre certaines limites, il n’en résulte pas la nécessité de choisir Alcyone pour ce centre, attendu qu’il pourrait bien tomber sur quelqu’autre étoile située dans le groupe ou dans son voisinage. Mais outre que c’est tout près de là que se trouve le groupe le plus brillant et le plus riche en étoiles de tout le ciel, et qu’il ne s’agit point ici d’un point arbitraire situé dans le voisinage peu apparent et qui n’ait rien qui le distingue, il ne se trouve nul part, même dans la région voisine, une aussi exacte concordance des mouvements propres qu’ici, et ces mouvements correspondent mieux que tous les autres aux conditions établies plus haut. Or si l’on doit considérer ce groupe central, entre les étoiles également éloignées, on peut présumer que la plus brillante de beaucoup présente la plus grande masse. Outre cela Alcyone, considérée optiquement, est au milieu du groupe des Pleïades; et son mouvement propre, déterminé par Bessel, est plus exactement en accord avec la moyenne de ceux des autres Pleïades; ainsi que des étoiles de cette région jusqu’à 10° de distance. Je puis donc établir comme conséquence de tout ce qui précède, que le groupe des Pleïades est le groupe central de l’ensemble du système des étoiles fixes, jusqu’aux limites extérieures déterminées par la Voie Lactée; et que Alcyone est l’étoile de ce groupe qui paraît être, le plus vraisemblablement, le vrai Soleil central.”
[19] See the article On Gravitation, Penny Cyclopædia, from the pen of the Astronomer-Royal.
[20] Delambre dates the commencement of modern astronomical observation in its most perfect form from Maskelyne, who was the first who gave what is now called a standard catalogue (a.d. 1790) of stars; that is, a number of stars observed with such frequency and accuracy, that their places serve as standard points of the heavens. His suggestion of the Nautical Almanack, and his superintendence of it to the end of his life, from its first publication in 1767, are mentioned in the Almanack (vol. i. p. 364); his Schehallion Experiment on Attraction in vol. iii. p. 69; and the character of his Greenwich Observations in Greenwich Observatory in vol. ii. p. 442.
[21] Experiments to determine the Density of the Earth. By Henry Cavendish, Esq., F.R.S. and F.A.S.—Philosophical Transactions, 1798.
[22] Adams: An Explanation of the observed irregularities in the motion of Uranus, on the hypothesis of disturbance caused by a more distant Planet.—Appendix to Nautical Almanack for 1851.
[23] Le Verrier: Premier Mémoire sur la théorie d’Uranus, Comptes Rendus, vol. xxi.; Sur la planête qui produit les anomalies observées dans le mouvement d’Uranus.—Ib. vol. xxiii.
[24] The experiment alluded to is one of a series by M. Plateau, who thus describes his arrangement of the fluid:—“We begin by making a mixture of alcohol and distilled water, containing a certain excess of alcohol, so that when submitted to the trial of the test tube it lets the small sphere of oil fall to the bottom rather rapidly. When this point is obtained, the whole is thrown upon filters, care being taken to cover the funnels containing these last with plates of glass; this precaution is taken in order to prevent, as much as possible, the evaporation of the alcohol. The alcoholic liquor passes the first through the filters, ordinarily carrying with it a certain number of very minute spherules of oil When the greater part has thus passed, the spherules become more numerous; what still remains in the first filters, namely, the oil and a residue of alcoholic liquor, is then thrown into a single filter placed on a new flask. This last filtration takes place much more slowly than the first, on account of the viscosity of the oil; it is considerably accelerated by renewing the filter once or twice during the operation. If the funnel has been covered with sufficient care, the oil will collect into a single mass at the bottom of the flask under a layer of alcoholic liquor.”—On the Phenomena presented by a free Liquid Mass withdrawn from the action of Gravity. By Professor Plateau, of the University of Ghent. Translated from the Memoirs of the Royal Academy of Brussels, vol. xvi.; in the Scientific Memoirs, vol. iv. part 13.