Second Report of the British Association. 459 



" Of pendulum experiments, the most valuable series is that made by 

 Captain Sabine in almost every practicable latitude. Invariable pendulums 

 which had been observed in London (to ascertain the number of vibrations 

 made per day,) were observed in the same manner at all the stations, and 

 again in the same manner on returning to London. In this manner, without 

 ascertaining the absolute force of gravity at any one place, the proportion at 

 different places is found probably with greater accuracy than by any other 

 method. This is the method commonly adopted by the English experimen- 

 ters. Experiments were previously made at several places in Britain by 

 Captain Kater; and others have been made in different parts of the world 

 by Captain Hall, Sir Thomas Brisbane, Mr. Goldingham, &c. A vast num- 

 ber of most careful observations by Captain Foster, in his last voyage, has 

 been received in England, and is now (I believe) preparing for the press. 

 Advantage has also been taken of our repeated expeditions to the North 

 Seas to observe pendulums at high latitudes. The method commonly used 

 by the French philosophers was, to observe the absolute length of the se- 

 conds pendulum at each station : thus they experimented at several stations 

 in France and Italy, in the Mediterranean, and in Britain. An extensive 

 series, however, made in Freycinet's voyage, and a few in Duperrey's, were 

 made with invariable pendulums. In the course of experiments for ascer- 

 taining the absolute length of the seconds pendulum by a new method, Bes- 

 sel found that the correction applied in all former experiments for the buoy- 

 ancy of the air was defective. This has been fully confirmed by Captain 

 Sabine's experiments in a vacuum ; and Mr. Baily has been actively em- 

 ployed in determining, with superior accuracy, the correction that ought to 

 be sdopted. This error, however, produces very little effect on the deter- 

 minations of the proportion of the force of gravity at different places. 



" A series of pendulum experiments was made by Carlini, at the Hospice 

 of Mont Cenis, to ascertain the diminution of gravity at the height of a 

 thousand toises. The account of these is given in the Milan Ephemeris for 

 1824. The result obtained for the mean density of the earth agrees pretty 

 well with that generally received j but the changes which experiment has 

 shown to be necessary in the elements of reduction, throw a little doubt 

 upon its value. The mountain Schehallien (on which Maskelyne's obser- 

 vations of attraction were made,) has been surveyed, and some alteration 

 made in the numerical results : the calculations of Cavendish's experiments 

 have also been corrected. See various volumes of the Phil. Trans. 



** In the theory, no improvement has been made, I believe, since the time 

 of Clairaut. No satisfactory rule has been given for taking into account the 

 elevation of the station : perhaps the considerations suggested by Dr. Young 

 in the Phil. Trans, for 1819, may be regarded as the most useful. 



" It is generally thought that the measures of arcs give an ellipticity of 

 nearly ^-^ to the earth ; some persons considering it a little greater, and others 

 a little smaller. The pendulum experiments, with Clairaut's theorem, give 

 an ellipticity rather greater, though not without remarkable anomalies." 



The interest and importance, as well in a national as in a scien- 

 tific point of view, of the tenth section of this Report, are so consi- 

 derable, that we feel we should omit a duty to our readers were we 

 not to transfer it entire to our pages. 



" X. In the preceding sections I have endeavoured to give materials for 

 estimating the steps which Astronomy has made in this century, and for 

 understanding its present state, at least in all the important parts. But I 

 cannot forget that the Association which I have the honour to address, while 

 it is a Philosophical Association, is also a British Association, and that while 

 k is anxious to promote science abstractedly, it is also jealous of our na- 



3N2 



