CHAP. III., 4.] ASTRONOMY. BAILY COLONEL EVEREST M. STRUVE. 



Reduction 

 to a va- 

 cuum. 



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 Philosophical Transactions for 1832. 

 He vibrated pendulums of different forms in vacua 

 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 

 Mr Stokes, 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 



(247.) 

 Investi- 

 gated by 

 Besscl and 

 others. 



(248.) 



lesult as experiments of Captain Foster in both hemispheres 

 oblateness. %&& ^ or *^ e ear th's compression. Colonel Sabine had 

 found it jj^jQ 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 d * e *P e " 

 the earth's density, has been mentioned in a previous 

 section. 



In conclusion of the subject of the figure of the (249.) 

 earth deduced from observation, I shall now briefly Rec n * 

 refer to the results of geodetical measures more re- measures : 

 cent than those noticed in the first section of this Colonel 

 chapter, which comprehended the great French arc. Everest 

 In a matter so much of detail, which will be bet- Ml St 

 ter appreciated from special articles in the Encyclo- 

 pcedia, 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 EVE- 

 REST, the second by M. STRUVE, the eminent astrono- 

 mer of Pulkowa. 



The measurement of an arc of the meridian in In- (250.) 



dia by Colonel Lambton in the early part of this cen- La " lb ^ on 



J -, , ,. , . J r . . and Eve- 



tury, has been mentioned in a tormer part of this rest ' s j n _ 



Dissertation, as well as in Sir John Leslie's. But its dian arc. 

 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 arc of Lambton, extending from Punnse 

 (Lat. 8 9' 35") to Daraargida (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 ^f 

 measurement of an arc extending from Damargida, which it 

 where Lambton's arc terminated, to Kaliana (Lat. was exe- 

 29 3(y48"), a space of 797 miles, covering an arc cuted ' 

 of 11 27' 33", 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- 



