1855.] Pendulum-experiments at Harton Collierr, 17 



WEEKLY EVENING MEETING, 



Friday, February 2. 



Sir Henry Holland, Bart. M.D. F.R.S. 

 Vice-President, in the Chair. 



G. B. Airy, Esq. F.R.S. Astronomer-Royal, 



On the Pendulum-experiments lately/ made in the Harton Colliery, 

 for ascertaining the mean Density of the Earth. 



The speaker commenced with remarking that the bearing of the 

 experiments, of which he was about to give a notice, was not 

 limited to their ostensible object, but that it applied to all the 

 bodies of the Solar system. The professed object of the experiments 

 was to obtain a measure of the density of the earth, and therefore 

 of the mass of the earth (its dimensions being known) ; but the 

 ordinary data of astronomy, taken in conjunction with the laws 

 of gravitation, give the proportions of the mass of the earth to 

 the masses of the sun and the principal planets ; and thus the 

 determination of the absolute mass of the earth would at once 

 give determinations of the absolute masses of the sun and 

 planets. To show how this proportion is ascertained, it is 

 only necessary to remark, that a planet, if no force acted on it, 

 would move in a straight line ; that, therefore, if we compute 

 geometrically how far the planet moves in a short time, as an hour, 

 and then compute the distance between the point which the planet 

 has reached in its curved orbit, and the straight line which it has 

 left, we have found the displacement which is produced by the sun*s 

 attraction, and which is therefore a measure of the sun's attraction. 

 In like manner, if we apply a similar calculation to the motion of a 

 satellite during one hour, we have a measure of the attraction of its 

 primary. The comparison of these two gives the proportion of the 

 attraction of the sun, as acting upon a body, at one known distance, 

 to the attraction of a planet, as acting upon a body at another known 

 distance. It is then necessary to apply one of the theorems of the 

 laws of gravitation, namely, that the attraction of every attracting 

 body is inversely as the square of the distance of the attracted body ; 

 and thus we obtain the proportion of the attractions of the sun and 

 a planet, when the bodies upon which they are respectively acting 

 are at the same distance from both : and finally, it is necessary to 

 apply another theorem of the law of gravitation, namely, that tlie 

 attractions thus found, corresponding to equal distances of the 



Vol. II. c 



