Reduction 
to a va- 
cuum, 
(247.) 
Investi- 
gated by 
Bessel and 
others. 
Mr Stokes. 
(248.) 
Result as 
to Earth's 
oblateness. 
Cuar, IIL. § 4.] ASTRONOMY.—BAILY—COLONEL EVEREST—M. STRUVE. 853 
the air’s buoyancy. The mere resistance of the air 
to the motion of the pendulum may be shown to pro- 
duce opposite and compensating effects in the descend- 
ing and ascending semi-vibrations: but the presence 
of air, as of any other fluid, diminishes the total ef- 
fect of gravity on the bob, and that in a degree de- 
pending on the density of the air or atmospheric 
pressure. But besides this obvious “ reduction to a 
vacuum,” it appears that the air acts also in a dif- 
ferent way. Owing to its inertia and cohesion a 
portion of air is shoved along with the pendulum, 
or otherwise put in motion by it, and the force 
thus spent in moving air is not compensated during 
the two semi-vibrations. This effect was clearly as- 
certained and measured by Dubuat about 1786, and 
described in the second edition of his “ Principes d’Hy- 
draulique ;” but it appears to have been totally un- 
observed by his own countrymen and others, who 
(like Borda) were soon after actively engaged with the 
same subject. The effect was experimentally re-dis- 
covered by Bessel, and made the subject of an admi- 
rable series of experiments by Baily, which were pub- 
lished in the Philosophicat Transactions for 1832. 
He vibrated pendulums of different forms in vacuo 
and in air (as Colonel Sabine had done to a more 
limited extent shortly before), and he ascertained the 
corrected “ reduction to a vacuum” to be in many in- 
stances double of the old correction, and to depend 
materially on the form and density of the pendulum, 
It even appeared that when a convertible pendulum 
is swung in two positions in the manner of Kater, 
the correction is different in the two cases, owing to 
the want of symmetry, so that a pendulum convert- 
ible in air of a given density is not convertible in va- 
cuo, nor vice versa, 
Bessel and Baily agree in imputing the effect to 
the clinging of a mass of air to the metallic pendu- 
lum, thus rendering it in effect a pendulum of much 
less density, for which the ordinary correction for 
buoyancy will have to be increased. This view is 
probably rather a popular than an exact expression of 
the fact, and the correction need not be always pro- 
portional to that of buoyancy. Poisson has treated 
the question mathematically, and Professor Stokes 
has recently resumed the whole subject, both mathe- 
matically and practically, and considers that he has 
arrived at precise mathematical results in several 
cases. The result may probably be a revival of inte- 
rest in pendulum observations, which has manifestly 
very much declined since the existence of this irregu- 
larity has been fully established, and the incompe- 
tency of existing rules for “ reduction to a vacuum” 
clearly shown. 
In 1833, Baily deduced from the very elaborate 
experiments of Captain Foster in both hemispheres 
zee for the earth’s compression. Colonel Sabine had 
found it ,4,, from his own observations. The French 
and Russian observers concur in obtaining a result 
sensibly greater. Of these experiments, Captain Fos- 
ter’s is by much the most extensive. Mr Baily’s re- The Caven- 
petition of Cavendish’s experiment for determining “is expe 
the earth’s density, has been mentioned in a previous meter 
section, 
In conclusion of the subject of the figure of the  (249.) 
earth deduced from observation. I shall now briefly ve : 
refer to the results of geodetical measures more re- , ag 
cent than those noticed in the first section of this —Colonel 
chapter, which comprehended the great French are, Everest— 
In a matter so much of detail, which will be bet-™ 5%" 
ter appreciated from special articles in the Encyelo- 
pedia, I shall best fulfil the ends of this discourse, 
by merely directing attention to the two most con- 
spicuous and important of these measurements of 
the earth, one in India, the other in central and 
northern Europe ; the first directed by Colonel Evz- 
rest, the second by M. Srruve, the eminent astrono- 
mer of Pulkowa. 
The measurement of an are of the meridian in In-_ (250.) 
dia by Colonel Lambton in the early part of this cen- pence 
tury, has been mentioned in a former part of this post's In- 
Dissertation, as well as in Sir John Leslie’s, But its dian are. 
value has been prodigiously increased by the exten- 
sion of the same by Colonel Everest, who was at one 
time the principal assistant and coadjutor of Colonel 
Lambton. The are of Lambton, extending from Punnze 
(Lat. 8° 9’ 35”) to Damargida (Lat. 18° 3’ 15”), was 
measured after the model of the English trigonome- 
trical survey, by means of a 100-feet steel chain for 
the base, a 3-feet theodolite for the azimuths, and a 
5-feet zenith sector for latitudes. Colonel Everest 
pursued the triangulation in a northerly direction for 
some years after the death of Lambton in 1823; 
but losing his health, and being forced to return to 
Europe, he was provided, by the princely munificence 
of the East India Company, with an entirely new set 
of instruments, and with two able assistants, Cap- 
tains Waugh and Renny. 
He resumed operations in 1832, which were con- _ (251.) 
tinued until 1840, and included an entirely new ee 
measurement of an are extending from Damargida, which it 
where Lambton’s are terminated, to Kaliana (Lat, was exe- 
29° 30’ 48”), a space of 797 miles, covering an are °%*¢- 
of 11° 2733”. The latest geodetical improvements 
were introduced. The bases were measured by Ge- 
neral Colby’s compensating bars, in which the effect 
of temperature is self-corrected. The signals from 
station to station were made during the day by he- 
liotropes reflecting the solar ray parallel to the sides 
of the triangles to be measured. The azimuths were 
determined by a 3-feet theodolite of Troughton, in 
addition to that of Carey used by Lambton; the 
astronomical observations were made with two circles 
of large dimensions. The difficulties and annoyances 
experienced in conducting so vast a work, requiring 
such excessive accuracy, in so remote a situation, 
can only be judged of by referring to Colonel Eve- 
rest’s graphic details contained in his elaborate ac- 
