(153.) 
The Nau- 
tical Alma- 
nac. 
(154.) 
Attraction 
of moun- 
tains. 
834 
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. 
But, for the application of the method of lunar 
distances to navigation, farther aid than the con- 
struction of good 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 (Connatssance 
des Tems) for a considerable number of years. 
II. The determination of the Attraction of Sche- 
hallien, and of the Earth’s Density——The deviation 
of the plumb-line from the vertical by the neighbour- 
hood of a mountain had been pointed out by Newton! 
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.? 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 
ASTRONOMY.—MASKELYNE—DELAMBRE. 
[Diss, VI. 
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 Mask = 
in the latitude of Schehallien corresponds to 42-94 phates 
of latitude. The observed difference of latitude by at Schehal- 
337 observations with Sisson’s 10-feet zenith sector lien. 
was 54”6.2 The excess, or 11”: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 
r7rkoz 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 
yvsz) 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- 
Earth’s 
density de- 
duced, 
(156.) 
Michell 
and Caven- 
dish’s expe- 
riment for 
the same 
end. 
1 De Mundi Systemate, § 22. Newton, in a very remarkable passage of the Third Book of the Principia (Prop. X.), con- 
jectures that “the quantity gf matter in the earth may be five or six times greater than if the whole were composed of water.” 
® A laughable mistake of Zach in his account of the Schehallien experiment (in his Ater 
on by Playfair in the Edinburgh Review. 
des Montagnes) is commented 
In a note to the word Schehallien, Zach says, “ Montagne appelée 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 
ougss to have delivered him, for though he could not learn there what was Erse, he might have learned what was English.” 
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, 
