Supplement to the fourth, fifth and sixth editions of the Encyclopaedia Britannica > Volume 1

644
Attraction
II ,
Atwood.
attraction.
In the preceding investigations, we have followed the
method of Maclaurin for points situated in the surface
oi a spheroid, or within the solid. Ihis method has
always been justly admired ; but neither its inventor,
nor, as far as we know, any other Geometer, has ap¬
plied it, excepting to spheroids of revolution; and it
is here for the first time, extended to ellipsoids. In
regard to points without the surface, we have employ¬
ed the method first given by Mr Ivory, m the Phloso-
vhical Transactions for 1809. The combination ot
these two methods has enabled us to derive the attrac¬
tions of an ellipsoid on a point placed anywhere, from
the attractions at the poles. Thus, this extremely com¬
plicated problem has, by geometrical reasoning of no
great difficulty, been reduced to the investigation ot
the polar attractions, which are the only cases that re¬
quire a direct computation.
S4-. Of the attractions of spheroids composed of el- Attraction
Uptical shells that vary in their densities and figures AtJ')0(}<
according to any laiu.
When a spheroid is composed of concentric el¬
liptical shells of variable density and figure, we may
consider every shell as the difference of two homoge¬
neous spheroids of the same density with the shells, and
having their surfaces coinciding with the surfaces of the
shell. The attractions of the spheroids being computed
by the preceding methods, their difference will be equal
to the attractions of the shell; and the integral ob¬
tained by summing the attractions of all the shells,
will give the attractions of the heterogeneous sphe¬
roid. This case, therefore, gives rise to no new diffi¬
culties, except what are purely mathematical, and de¬
pend upon the law, according to which, the densities
and figures of the shells are supposed to vary, (cc.)
ATTRACTION OF MOUNTAINS. See
Mountains, Attraction of, in the Encyclopaedia,
and in this Supplement.
ATWOOD (George), an Author celebrated tor
the accuracy of his mathematical and mechanical in¬
vestigations, and considered as particularly happy in
the clearness of his explanations, and the elegance of
his experimental illustrations, wras born in the early
part of the year 1746. He was educated at West¬
minster school, where he was admitted in 17 59- Six
years afterwards he was elected off to Iiinity Col¬
lege, Cambridge. He took his degree of Bachelor
of Arts in 1769, with the rank of third wrangler, Dr
Parkinson, of Christ’s College, being senior of the
year. This distinction was amply sufficient to give
him a claim to further advancement in his own Col¬
lege, on the list of which he stood foremost of his
contemporaries; and, in due time, he obtained a fel¬
lowship, and was afterwards one of the tutors -of the
College. He became Master of Arts in 1772 ; and,
in 1776, was elected a Fellow of the Royal Society
of ^London.
The higher branches of the Mathematics had, at
this period, been making some important advances at
Cambridge, under the auspices of Dr Waring, and
many of the younger members of the University be¬
came diligent labourers in this extensive field. Mr
Atwood chose, for his peculiar department, the illus¬
tration of mechanical jmd experimental philosophy,
by elementary investigations and ocular demonstra¬
tions of their fundamental truths. He delivered, for
several successive years, a course of lectures in the
Observatory of Trinity College, which were very ge¬
nerally attended, and greatly admired. In the year
1784, some circumstances occurred which made it
desirable for him to discontinue his residence at Cam¬
bridge ; and soon afterwards Mr Pitt, who had be¬
come acquainted with his merits by attending his lec¬
tures, bestowed on him a patent office, which re¬
quired but little of his attendance, in order to have
a claim on the employment of his mathematical abi¬
lities in a variety of financial calculations, to which
he continued to devote a considerable portion of his
time and attention throughout the remainder of his
life. *
The following, we believe, is a correct list of Mr
Atwood’s publications:
1. A Description of Experiments to illustrate a
Course of Lectures. 8vo, About 1775, or 1776.
2. This work was reprinted with additions, under
the title of An Analysis of a Course of Lectures on
the Principles of Natural Philosophy. 8vo. Cambr.
1784. ' „ ,
3. A General Theory for the Mensuration of the
Angle subtended by two objects, of which one is observ¬
ed by Rays after two Refections from plane Surfaces,
and the other by Rays coming directly to the Specta¬
tor’s Eye. Phil. Irans. 1781, p. 3Q5.
4. A Treatise on the Rectilinear Motion and Ro¬
tation of Bodies, with a Description of Original Ex¬
periments relative to the Subject. 8vo. Cambr. 1 / 84.
5. Investigations founded on the Theory of Motion,
for determining the Times of Vibration oj Watch Ba¬
lances. Phil. Trans. 1794, p. H9» .
6. The Construction and Analysis of Geometrical
Propositions, determining the positions assumed by
homogeneal bodies, which foot freely, and at fpf
on afuid'ssurface; also Determining the Stability
of Ships, and of other Floating Bodies. Phil. Trans.
1796, p. 46. ,
7. A Disquisition on the Stability of Ships. 1 hil.
Trans. 1798, p. 201.
8. A Review of the Statutes and Ordinances of As¬
size, which have been established in England from the
Uh year of King John, 1202, to the 31th of his pre¬
sent Majesty. 4to, Lond. 1801.
9. A Dissertation on the Construction and Proper¬
ties of Arches. 4to, Lond. 1801.
10. A Supplement to a Tract entitled a Treatise on
the Construction and Properties of Arches, published
* See Literary Memoirs of Living Authors, 2 vol. 8vo, Lond. 1798; Morning Herald, 17th July
1807; Nichols’s Literary Anecdotes of the Eighteenth Century, Vol. VIII. 8vo, Lond. 1814.