﻿On Barometric Differences and Fluctuations. 429 



tical value. The critical distance is one of unstable equilibrium. 

 For the case of p = (that is to say, the free rigid body a mass- 

 less sphere) the critical distance is 



, or 2-251r; 



x/ 1 -^ 1 



3 



that is, very approximately 2£ radii of the free globe. 



Yours truly, 

 Professor Guthrie. William Thomson. 



11th Jan. 1871. 



Sir, — I am desired by Sir W. Thomson to send you a de- 

 scription of the following experiment on " Approach caused by 

 Vibration." 



A light collodion balloon is filled with hydrogen, and the 

 thread which closes the orifice is prolonged to a length of about 

 40 inches. To the free end of the thread a light piece of wood 

 is tied to serve as a floater. The latter is now put upon the sur- 

 face of water and serves to moor the balloon, giving it at the 

 same time freedom of motion in all directions. If, now, a sheet 

 of tin-plate (or, better, a light wooden board with rounded edges) 

 is held perpendicularly in a line with the centre of the balloon 

 and vibrated to and fro at a distance of several inches from it, 

 the balloon approaches the vibrating plate*, drawing with it the 

 floater over the surface of the water. Vibrations of the bal- 

 loon isochronous with those of the plate are also observed. 

 I am, Sir, 



Your obedient Servant, 



Professor Guthrie. John Tatlock. 



LVI. On Barometric Differences and Fluctuations. By John 

 Knox Laughton, M.A., F.R.A.S., F.R.G.S., Mathematical 

 and Naval Instructor at the Royal Naval College y Portsmouth. 



[Continued from p. 350.] 

 Part III. The Barometric Depression in Storms. 



MANY years ago Admiral Fitzroy pointed out that storms 

 are properly to be regarded merely as deviations from the 

 ordinary and regular movements of the atmosphere, and that, 

 violent as is their force, disastrous as are their effects, they con- 

 stitute in reality a very small part of that motion of the air which 

 is continually going on, and which the study of prevailing winds 



* Compare with Herr Schellbach's experiments, sup. p. 422, who, on 

 using a tuning-fork of 512 single vibrations per second, obtained repul- 

 sion.— F. G. 



