54 



MATHEMATICAL AND PHYSICAL SCIENCE. 



[Diss. VI. 



(240.) 

 Captain 

 Kater ; 



his con- 

 vertible 

 pendulum. 



(241.) 

 His Colli- 

 mator. 



(242.) 

 Francis 

 Baily; 



his early 

 history 



and Baily. As regards the determination of the 

 Earth's figure, the relative length of a pendulum vi- 

 brating seconds in different latitudes is all that is re- 

 quired ; but the absolute length in a given place is 

 necessary for finding the force of gravity there ex- 

 pressed by the acceleration of falling bodies in vacuo 

 in one second ; and as the British standard yard was 

 at that time made to depend on the length of the pen- 

 dulum vibrating seconds at London, this enquiry was 

 mixed up with the intricate one of National Standard 

 Measures. 



CAPTAIN HENBY KATER was a person of remarkable 

 mechanical skill and ingenuity, and his numerous pa- 

 pers in the Philosophical Transactions, between 1813 

 and 1828, refer chiefly to the accurate construction 

 and use of the Pendulum, the Balance, and certain As- 

 tronomical Instruments. He first applied to practical 

 use the beautiful theorem of Huygens respecting the 

 convertibility of the Centres of Oscillation and Sus- 

 pension of the pendulum. A metallic bar pendulum 

 was provided with two parallel knife-edges facing op- 

 posite ways, and upon either of which it might be 

 swung. They were so arranged that when either was 

 used as the point of suspension, the other nearly re- 

 presented the centre of oscillation, and by means of 

 a small adjustable weight, this condition might be ac- 

 curately fulfilled ; in which case (by Huygens' princi- 

 ple) the distance between the knife-edges corresponded 

 accurately to the length of an equivalent simple pen- 

 dulum. This elegant application was farther tested 

 by Mr Baily, but it was found liable to serious prac- 

 tical objections. Baily dispensed with the sliding 

 weight, and rendered the pendulum invariable : the 

 distance of the knife-edges was adjusted by grinding. 



To Kater we owe the invention of the floating Col- 

 limator, for ascertaining the accurate zero or level 

 points of divided astronomical instruments. The op- 

 tical principle on which it depends is a very beau- 

 tiful one, and the invention of Kater, with several 

 improvements in point of form, has become the auxi- 

 liary of nearly every observatory in the world. It is 

 one of those small but happy improvements which 

 affect materially the progress of Science. 



FRANCIS BAILY, who was born 28th April 1774, and 

 who died 30th August 1844, was an instance of a 

 person not endowed with extraordinary talents, yet of 

 singular use in his generation. Those who knew Mr 

 Baily personally, and what he accomplished for Astro- 

 nomy, are best aware how rare and yet how estimable 

 are such qualifications as his. Born of parents of a 

 middle rank of life, educated at a country school, and 

 at an early age thrown into the vortex of commer- 

 cial life in the city of London, no probability could 

 have appeared more remote than that he should ac- 

 quire the respect of even the most accomplished As- 

 tronomers by his scientific acquirements and perform- 

 ances. He enjoyed the advantage, in the first place, 



of a perfectly sound and uninterrupted state of health 

 until his 7 Oth year. His intellect partook of the 

 equable tone of his bodily powers. Method, and an and charac- 

 ardour which, instead of acting by impulses, seemed ter - 

 inexhaustible and unvaried, were the secrets of his 

 success. Mr Baily never was hurried, and he never 

 was unemployed. He had always leisure to encourage 

 men of like tastes, and he was never unwilling to learn 

 from any one who had knowledge to communicate. 

 To do one thing at a time was his first principle. 

 When thoroughly accomplished, it was completely put 

 aside, and a fresh subject undertaken. Whether an 

 experiment or a calculation was found to require a 

 month or three years, it was pursued to a successful 

 completion, without stint or grudging. No amount 

 of contrariety and failure in such matters was ever 

 known to ruffle his temper, or to make his persever- 

 ance falter. 



Francis Baily was known, in the first place, as the 

 founder (in 1820) of the Astronomical Society of 

 London, an institution of signal utility, which has nomical 

 ever since been conducted with great judgment and Society, 

 success. His accurate knowledge of past and current 

 astronomical history, and his personal acquaintance 

 with most European astronomers, enabled him to 

 conduct its affairs with great advantage. 



Amongst his other acquirements, he studied his- (244.) 

 tory and chronology with his customary precision. A Writings 

 rery able paper on the date of the celebrated Eclipse " d g^ ' 

 of Thales was his earliest scientific publication, 1 and Catalogues, 

 the literature and phenomena of solar eclipses occu- 

 pied his particular attention ever afterwards. His ac- 

 counts of the Eclipse of 1820, of the Annular Eclipse 

 of 1836, which he observed at Jedburgh, and the Total 

 Eclipse of 1842, with its marvellous revelation of the 

 rose-coloured protuberances of the solar atmosphere, 

 are among the most interesting and classical of his 

 writings. He promoted greatly the formation and 

 publication of accurate Star Catalogues, and he in- 

 vented, independently of Bessel, constant numbers 

 for facilitating the calculation of Precession, Nuta- 

 tion, and Aberration, as affecting the places of parti- 

 cular stars. He also published a most useful collec- 

 tion of Astronomical Tables and Formulae, and an 

 elaborate and very curious life of Flamsteed. 



I shall now proceed to notice his experiments on 

 the Pendulum, which are more particularly connected 

 with the present section. 



Mr Baily's most important observations were on . 

 a correction of the motion of a pendulum long mis- 

 understood. The corrections usually admitted were observa- 

 the following: 1. For temperature altering the tlon3 - 

 length of the pendulum. 2. For the height of the 

 station above the level of the sea. Dr Young showed 

 that this correction was commonly overrated, owing 

 to the sensible amount of the attraction of the ele- 

 vated land on which the experiment is made. 3. For 



(245.; 



(246.) 

 Baily's 



1 Phil. Trans. 1811. 



