it was thought that but for such motion all celestial objects must have collapsed and formed a chaos. There are some stars which seem to be in a great measure out of the reach of the attractive force of other stars, whence Sir W. Herschel was disposed to consider them as probably centres of extensive systems like our own. Among them, with probably many others, are :— Vega (a Lyræ). Capella (a Auriga). Arcturus (a Boötis). Sirius (a Canis Majoris). Canopus (a Argûs). Markab (a Pegasi). Bellatrix (y Orionis). Menkab (a Ceti). Algorab (8 Corvi). The twinkling, or scintillation, of the stars is a phenomenon which requires to be briefly noticed. The effect is too well known to need description, but the cause is involved in much obscurity, though referred by most observers to the interference of light". Many ascribe it more immediately to the varying refrangibility of the atmosphere, and this latter theory has much to recommend it. A quiescent condition of the air is unfavourable to the development of twinkling, and in general the phenomenon is more marked at the surface of the Earth than at high elevations where the air is more rarefied-both of which facts point out the atmosphere as an influential agent. Humboldt mentions that in the pure air of Cumana twinkling ceased after the stars attained an elevation of 15° above the horizon. According to Dufour, red stars twinkle less than white ones. Liandier, from repeated observation, says that he is convinced that twinkling is due to disturbances of the atmosphere, brought about by winds and currents of air. The greater the twinkling, the easier it is to see faint stars. Scintilla, a spark of fire. Eng. Cycl., Arts and Sciences Div., Art. Twinkling. b Month. Not. R.A.S., vol. xviii. p. 51. CHAPTER IV. CLUSTERS AND NEBULE. Arranged in three classes.-Five kinds of Nebula.-The Pleiades.-The Hyades.Mentioned by Homer.-Præsepe.-Opinion of Aratus and Theophrastus.—Coma Berenicis.- List of Clusters. Annular Nebula. - Elliptic Nebulæ. - Spiral Nebula.-Planetary Nebula.- -Nebulous stars.-List of irregular Clusters.Notes to the objects in the list.-The Nubecula major and minor.-List of Nebula in Sir J. Herschel's Catalogue of 1864. - Historical statement relating to the observation of Nebula and Clusters. F we examine the heavens on a clear evening when the I'Moon is not shining, we shall find here and there groups of stars which seem to be compressed together in such a manner as to present a hazy cloud-like appearance; these are termed clusters and nebula, and are usually classed as follows: 1. Irregular groups, visible more or less to the naked eye. telescope. 3. Nebulæ, for the most part irresolvable with the telescopes which we at present possess. The objects forming the 3rd class are in their turn subdivided into i. Annular nebulæ. ii. Elliptic nebulæ. iii. Spiral nebulæ. iv. Planetary nebulæ. v. Nebulous stars. Of the 1st class there are several examples to be found, with |