ON THE MEASUBEMENT OF THE LUNAU DISTURBANCE OF GBAVITY. 95 



Second Report of the CoviTtiittee, consisting of Mr. Gr. H. Darwin, 

 Professor Sir William Thomson, Professor Tait, Professor Grant, 

 Dr. Siemens, Professor Purser, Professor Gr. Forbes, aoid Mr. 

 Horace Darwin, appointed for the Measurement of the Lunar 

 Disturbance of Gravity. Written by Mr. Gr. H. Darwin. 



Shortly after the meeting of the British Association last year (1881), 

 the instrument with which my brother and I were experimenting at the 

 Cavendish Laboratory, at Cambridge, broke down, through the snapping 

 of the wire which supported the pendulum. A succession of unforeseen 

 circumstances have prevented us, up to the present time, from resuming 

 our experiments. 



The body of the present Report, therefore, will merely contain an 

 account of such observations by other observers as have come to our 

 knowledge within the past year, and it must be taken as supplementary to 

 the second part of the Report for 1881 . The Appendix, however, contains 

 certain theoretical investigations, which appear to me to throw doubt 

 on the utility of very minute gravitational observations. 



The readers of the Report for 1881 will remember that, in the course 

 of our experiments, we were led away from the primary object of the 

 Committee, namely the measurement of the Lunar Disturbance of Gravity, 

 and found ourselves compelled to investigate the slower oscillations of 

 the soil. 



It would be beyond the scope of the present Report to enter on the 

 literature of seismology. But, the slower changes in the vertical having 

 been found to be intimately connected with earthquakes, it would not 

 have been possible, even if desirable, to eliminate all reference to seismo- 

 logy from the present Report. 



The papers which are quoted below present evidence of a very mis- 

 cellaneous character, and therefore this Report must necessarily be rather 

 disjointed. It has seemed best in our account of work done rather to 

 classify together the observers than the subjects. This rule will, however, 

 be occasionally departed from, when it may seem desirable to do so. 



The interesting researches in this field made during the last ten years 

 by the Italians, are, I believe, but little known in this country, and as the 

 accounts of their investigations are not easily accessible (there being, for 

 example, no copy of the 'Bulletino' referred to below at Cambridge), it 

 will be well to give a tolerably full account of the results attained. I 

 have myself only seen the ' Transactions ' for four years. 



The great extension which these investigations have attained in Italy 

 has been no doubt due to the fact of the presence of active volcanos and 

 of frequent sensible earthquakes in that country. But it is probable that 

 many of the same phenomena occur in all countries. 



In 1874 the publication of the ' Bulletino del Vulcanismo Italiano ' 

 was commenced at Rome under the editorship of Professor S. M. de 

 Rossi, of Rome.' As the title of this publication shows, it is principally 

 occupied with accounts of earthquakes, but the extracts made will refer 

 almost entirely to the slower oscillations of level. 



' I am compelled to make this abstract from manuscript notes ; but my papers 

 having become somewhat disarranged, I am not absolutely certain, in one or two 

 places, of the year to which the observations refer. 



