1835.] Observations on Visible Vibration. 367 



natural direction, prevented the vibrating force from bear- 

 ing it round beyond east and west and west and east. 



42. With impure mercury I found the motion of the 

 needle or wire much retarded, and on covering the surface 

 of clean mercury with a very thin coating of lycopodium 

 dust, the motion of the needle was altogether suspended. 



43. I found pieces of wood, cork, paper, cotton, wool, 

 camphor, camphor in a state of ignition, a silver amalgam, 

 &c, all to follow the same general law of revolving, con- 

 trary to the direction of the finger. 



44. I therefore, began to inquire, whether there might 

 not be opposing currents in the vibrating mercury, which 

 had hitherto escaped my notice, as indeed, was likely to 

 be the case, as the time that I could devote to this investi- 

 gation, out of an absorbing and anxious profession was so 

 limited, that I could only work by artificial light ; and I 

 afterwards found that in order to detect minute appearances, 

 a good natural light was indispensable. 



45. I need not now detail the various experiments which 

 led me to the conclusion, that what I had suspected, was 

 really the case : that in vibrating mercury there is a series 

 of concentric currents ; that the outer current revolves in the 

 direction of the finger, but there are inner currents revolving 

 in an opposite direction, the number or force of which, is 

 greater than those which revolve in the direction of the finger . 



46 In a second paper, I shall resume this subject. I 

 must, however, observe, that in performing these experi- 

 ments, the mercury should be perfectly clean and free from 

 amalgam ; and that in manipulating with the heated oils, 

 foot-glasses of about 3 inches diameter should be employed 

 and procured as thin as possible. The oils can be heated in 

 a Florence flask with the neck cut off to allow free evapo- 

 ration to the water contained in them. The glasses must 

 be heated first, and the oil poured in very gradually ; the 

 temperature is then to be noted, and the glass vibrated the 

 moment after. By this method, the glasses, if well an- 

 nexed, will bear a temperature of from 300° to 400°. It is 

 adviseable also to place the whole apparatus in a small tray, 

 in case of fracture either of the flask or glass. 

 Brown Street, Salisbury, 

 2Sth February, 1835. 



