[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. CHAPTER IV. MOLECULAR FORCES.