187:2.] OLDHAM AND MALLET CACHAR EARTHUITAKE. 257 



Although Dr. Oldham collected such facts with reference to over- 

 thrown objects, &c., as he deemed sufficient to enable him to apply 

 the same methods as those which the writer employed for fixing the 

 position and depth of the centre of impulse of the Neapolitan earth- 

 quake of 1857, and probably has, since the writer last heard from 

 him, actually worked out that problem, its full solution would be but 

 secondary in value to Dr. Oldham's observations as to the production 

 of vast earth-fissures, which are of the highest importance as afibrding 

 a better and more extended proof than has hitherto been presented 

 anywhere else of the fact first urged by the writer in his original 

 j)aper on the Dynamics of Earthquakes (Trans. Roy. Irish Acad. 

 1846), that earth-fissures, however vast, are only secondary pheno- 

 mena — that is, are not produced directly by the transit of the elastic 

 wave, but by the displacement of a portion of the surface-material 

 of the earth by resolved forces, due to the motions impressed upon it 

 by the wave. The observations by Dr. Oldham on the earth-fissures 

 produced in the plain of Cachar have been fix;ed by means of a set 

 of very fine photographs taken within a few weeks after the shock, 

 and transmitted by him to the writer. 



These are now laid before the Society ; and several are worthy of 

 an attentive study by everyone desirous of grasping through the eye 

 the exact mechanism of those most curious and formidable movements. 



It is time, however, to let Dr. Oldham speak for himself, through 

 the following extract of a letter received by the writer from him, 

 under date Calcutta, 29 March, 1869. 



" Calcutta, 29 March, 1869. 



" Now for earthquakes ; we had a sharp one on the 10th January, 

 1869. I had then just returned from Attok, and was too busy to go 

 off at once and look after it ; I did go in the beginning of February 

 up to Cachar, where it was reported to have been worst ; I went 

 there, and thence across the hills to Gunbulty. I could not get into 

 Munnepoor ; for, just at the time all the available carriage of the 

 country was taken up for an expedition against the Sooshuis, a tribe 

 of Kookies, who had been committing outrages on the southern fron- 

 tier of Cachar ; so I had to abandon that ; but at Cachar and other 

 places I succeeded in getting some good observations, taking your 

 noble Neapolitan earthquake as my guide. I think I have fixed the 

 locale of the seismic focus, the depth, the velocity of wave-particle, 

 tfec. ; I am just now waiting further measurements and information 

 from a few other points to which I could not myself get ; and you 

 shall have all particulars when they are brought into shape. There 

 happened to be a photograj)her up there at the time ; he was on 

 a professional tour ; and he seized the opportunity of being there to 

 take a series of views. They are not bad as photographs ; but he is 

 no artist,' and selected his points of view without any judgment ; 

 still it was a great piece of luck that he was there. I have a series 

 of these photographs specially procured for you ; they shall go by 

 the first steamer, a fortnight hence. The man sent them to me 

 moimted, so that I can't send them by post. 



" I quite long to send you aU particulars, and to see whether I 



