(230.) 
The new 
Greenwich 
Transit 
Circle. 
(231.) 
(234.) 
Figure of 
the earth 
resumed, 
850 MATHEMATICAL AND PHYSICAL SCIENCE. ' [Diss. VI. 
fixed stars, and then calculated the tabular places for 
the same instants of time. This last, especially in the 
case of the moon, was a truly formidable task. The 
whole labour was methodically performed at the 
expense of government by a staff of no less than six- 
teen computers, under Mr Airy’s vigilant superin- 
tendence, and it extended, first and last, over the 
space of a dozen years. The results, printed in the 
most compendious form, fill 2200 quarto pages. One 
consequence of this systematic reduction was, that 
MM. Leverrier and Adams were separately provided 
with the places and errors of the Tables of Uranus 
necessary for their great investigations; and the 
Lunar Theory has been enriched with Mr Hansen’s 
latest corrections through the same facilities. The 
final corrections of the co-efficients of the Lunar 
Tables are concisely stated by Mr Airy in a paper 
addressed to the Royal Astronomical Society. 
Like Bessel, Mr Airy is thoroughly acquainted 
with the theory and practice of mechanics ; and he 
has applied his knowledge both to the improvement 
of clocks and of proper astronomical instruments. A 
great and apparently successful experiment of the 
latter description is now taking place at Greenwich. 
A meridian circle (combining in effect a mural circle 
and transit instrument) has been erected, of great size 
and optical power (6 feet circle and 8} inch object- 
glass). But the novelty of the apparatus is its great 
weight and strength. Hitherto artists and astro- 
nomers have sought to avoid the evils of unsteadi- 
ness and flexure principally by the elaborate framing 
and trussing of light materials (chiefly brass tubes), 
and (in Germany especially) by an elaborate system 
of counterpoises. The new Greenwich circular in- 
strument is constructed of as few pieces as possible, 
of great strength and weight, and in great part of 
cast iron, The whole was elaborated and prepared 
for dividing in an engineering workshop, and with 
the powerful and accurate tools which modern engi- 
neers use. A man might stand upon the axis with- 
out producing the smallest bad effect. It is under- 
stood that the result is perfectly satisfactory as to 
steadiness and stiffness. Mr Airy has likewise in- 
vented and erected a new and most ingenious zenith 
tube (acting by reflection) for ascertaining the place of 
y Draconis, a star which, at its superior culmination, 
crosses the meridian of Greenwich almost in the zenith. 
There are few projects for the improvement of 
astronomy in the last twenty years in which Mr Airy 
has not been a counsellor or participator. He has Introduc- 
at all times given efficient aid to those engaged in the tion of gal- 
pursuit of science; and has been himself the first to imtoo fae 
port from America and to establish on a great scaleservation at 
the method of recording observations by means of a @reenwich. 
galvanic pressure produced on a sheet of paper sur- 
rounding a cylinder; which cylinder is moved in a per- 
fectly uniform manner by means of a connection witha 
clock producing uniform motion on the principle of 
the conical pendulum. By means of a simple contri- 
vance the tracer follows a slightly spiral line, so that 
one barrel may include many hours of continuous ob- 
servations. He has also established galvanically time 
signals, and employed them for the determination of 
longitudes. Besides all this, he has taken part in 
furthering geodetical measures, in regulating the 
national standards of length, and in promoting in 
various ways the collateral sciences. He contributed 
to the Encyclopedia Metropolitana a most able 
analysis of the methods for determining the Figure 
of the Earth, and his conclusions have been very 
generally adopted. In the same publication he has, 
given a theory of the Tides, already referred to (78, 
82). The public and his own university are indebted 
to him for several very valuable text-books on mathe- 
matical physics. 
During his incumbency at Greenwich, Magnetical _ (232.) 
and Meteorological Observations have been added pisguetion? 
to those of Astronomy, and engage the time of four teorologi- 
regular assistants in that department. The astro-cal obser- 
nomical assistants amount to six, besides occasional ¥442s- 
computers. It may safely be affirmed that there is 
not a public establishment in a more complete state 
of efficiency than is Greenwich Observatory at this 
moment; and that its traditional reputation, ex- 
pressed in the emphatic eulogium of Delambre, Art. 
(150), runs no risk of deterioration under the direction 
of Mr Airy. 
Our limits will not allow us to refer to the nu-  (233.) 
merous private observatories in England and public Other obs | 
ones abroad which are contributing useful elements 
to the promotion of astronomy. It is not too much 
to say that the activity of Greenwich has set a gene- 
rally good example. The observations at Oxford 
and Edinburgh have long been reduced on a syste- 
matic plan; those at Cambridge had been already so 
treated. The establishments where sidereal astro- 
nomy is particularly cultivated will be noticed in 
another section. 
§ 4. Borpa—Kater—Balty. The Figure of the Earth from Pendulum Observations—* Reduc- 
tion to a vacuum ;” Mr Stokes. Colonel Eynrest—M. Struve. Latest Measures of the 
Earth. M. Foucault’s Pendulum Experiment. 
In the first section of this chapter I have men- 
tioned the progress made in determining the figure 
of the earth by measurements of its surface in the 
early part of this century. I shall now advert 
briefly to the employment of the pendulum for the 
same purpose. 
Clairaut had published in 1743 a very remarkable ~ (235.) 
theorem which connects the force of gravity in any 
