36 



ASTKONOMY. MASKELYNE DELAMBRE. 



[Diss. VI. 



systematic reduction of all the lunar observations of 

 Maskelyne and Pond, and a comparison with Damoi- 

 seau's tables, under the direction of the present As- 

 tronomer Royal, Mr Airy. 



(153.) But, for the application of the method of lunar 

 The Nau- Distances to navigation, farther aid than the con- 

 tical Alma- gtruct } on O f g OO( j tables was required. This Maske- 

 lyne provided by obtaining the regular publication 

 of the Nautical Almanac, superintended by himself, 

 and containing the distances of the moon from the 

 principal fixed stars at predetermined hours for the 

 meridian of Greenwich, a comparison of which with 

 the distance observed by means of the sextant in any 

 part of the world enabled the seaman (after proper 

 reductions) to infer the exact Greenwich time of the 

 observation, and thence, by comparison with the local 

 time obtained by the usual methods, to obtain his 

 longitude. To this, long admitted to be the best 

 practical solution of the celebrated problem of " the 

 longitude at sea," Maskelyne contributed probably 

 more than any other person. His " Lunar Distances" 

 were reprinted in the French Almanac (Connaissance 

 des Terns) for a considerable number of years. 

 (154.) II. The determination of the Attraction of Sche- 



Attraction ^ a jr^ en> an & O f t ^ e EartKs Density. The deviation 

 tains. f the plumb-line from the vertical by the neighbour- 

 hood of a mountain had been pointed out by Newton 1 

 as a direct consequence, and also as a test, of the 

 principle that gravity resides in every part of the 

 earth as well as in the earth as a whole. Bouguer 

 had the merit of pointing out the form in which the 

 experiment might be made, and of making the trial, 

 though in a rude and insufficient manner, in the 

 Peruvian Andes in 1738. He observed the effect of 

 the mountain on the south side only, but at two sta- 

 tions unequally distant from its centre of attraction. 

 The numerical result being (as the author himself 

 admitted) without value, Maskelyne proposed to the 

 Royal Society in 1772 to repeat the observation on 

 some British mountain. A " Committee of Attrac- 

 tion" was named, which, besides Maskelyne, included 

 Cavendish, Franklin, and Horsley. Cavendish, as 

 might have been expected, took an earnest part in it. 

 The search for a suitable hill was confided to Mr 

 Charles Mason in 1773. Skiddaw and the York- 

 shire Hills were first thought of, but finally Sche- 

 hallien in Perthshire was preferred. 2 Thither Mas- 

 kelyne himself proceeded in 1774, with his assistant 

 Burrows, and by these two, with the aid of a local 

 land-surveyor, the labour of the astronomical and 



chief geodetical operations, including the measure- 

 ment of two base lines, was effected between the 30th 

 June and the 24th October, notwithstanding the 

 hindrances of a most unfavourable season. 



The distance between the two stations obtained with 

 Ramsden's 9-inch theodolite, was 4364-4 feet, which 

 in the latitude of Schehallien corresponds to 42"-94 

 of latitude. The observed difference of latitude by 

 337 observations with Sisson's 10-feet zenith sector 

 was 54"-6. 3 The excess, orll"-6, is the double at- 

 traction of the hill drawing the plumb-line towards 

 itself at the two stations. The sine of this angle, or 

 T7F(TT represents the actual ratio between the double 

 attraction of the hill and the attraction of the earth. 

 But by the computation of the attraction which the 

 hill ought to exert, from its figure, as determined by 

 Maskelyne's gauges, were its density the same as that 

 of the globe generally, this ratio should amount to 

 -5^3, which can only be accounted for by assuming 

 the earth to be denser on the average than the hill 

 of Schehallien in the proportion of 17804 to 9933. 

 This deduction was made by Dr Hutton by means of 

 a troublesome calculation of the summation of the 

 attractive effects of a number of vertical prisms into 

 which the hill was imagined to be divided. The arti- 

 fices of calculation were, however, due to Cavendish 

 (who it will be recollected was on the " Committee of 

 Attractions,") as Mr Airy ascertained from his manu- 

 scripts. A careful lithological survey of the hill 

 enabled Professor Playfair to deduce the probable 

 mean specific gravity of the globe to be between 4-56 

 and 4- 87, which was somewhat greater than Dr 

 Hutton assumed it. 



This is the proper place to mention an experiment 

 on the density of the Earth perhaps still more re- 

 markable, devised by the Rev. .Mr Michell, who con- 

 structed the apparatus, but first put in practice by 

 Mr Cavendish in 1797-8. It consisted in measuring 

 the force of gravitation between two spheres of such 

 small size that they could be moved by the hand 

 nearer to or farther from one another. The essen- 

 tial part of the invention was to contrive a balance 

 so delicate as to measure the almost inappreciable 

 tendency of such small bodies to unite. Newton had 

 shown that the attraction at the surface of any sphere 

 is directly as its radius, which he observed must 

 always be incomparably smaller than their tendency 

 towards the earth, that is, their weight. In the 

 largest and heaviest masses with which it has hither- 

 to been found practicable to operate, this tendency 



(155.) 

 Maske- 

 lyne's ob- 

 servations 

 at Schehal- 

 lien. 



Earth's 

 density de- 

 duced. 



(156.) 

 Michell 

 and Caven- 

 dish's expe- 

 riment foi 

 the same 

 end. 



1 De Mundi Systemate, 22. Newton, in a very remarkahle passage of the Third Book of the Principia (Prop. X.), con- 

 jectures that " the quantity of matter in the earth may be five or six times greater than if the whole were composed of water." 



2 A laughable mistake of Zach in his account of the Schehallien experiment (in his Attraction des Montagues) is commented 

 on by Playfair in the Edinburgh Review. In a note to the word Schehallien, Zach says, " Montagne appelee dans le pays en 

 langue Erse Maiden pap, qui veut dire orage perpetuel." It is needless to add that these two alleged synonyms are different in- 

 terpretations given by Gaelic scholars of the word. " From this inaccuracy," adds his reviewer, " his residence in London 

 ought to have delivered him, for though he could not learn there what was Erse, he might have learned what was English." 



3 Zach obtained the same result exactly by including all the observations, as Maskelyne had provisionally obtained by using 

 only those stars on which he most depended. 



