180 Mr. W. B. Croft on Breath Figures. 



The value of x for which (10) is a maximum may now be 

 found from Lagrange's interpolation formula. It is 



x = 'Q96, 

 corresponding to 



\ = 2aX7r/z=4-51x2a, .... (14) 



and agreeing with the value formerly obtained by a different 

 procedure. 



In like manner we get for the value of x giving maximum 

 instability in the case of the external fluid, 



.^ = •485, 

 and 



X=6-48x2a (15) 



Some numerical examples applicable to the case of water 

 were given in a former paper. It appeared that for a dia- 

 meter of one millimetre the disturbance of maxim*um insta- 

 bility is multiplied 1000-fold in about one-fortieth of a second 

 of time. This is for the case of internal fluid. If the fluid 

 were external, the amplification in the same time would be 

 more than one-million-fold. 



Terling Place, Witham, 

 July 2. 



XX. Breath Figures. 

 By W. B. Croft, M.A., Winchester College*. 



EIFTY years back Prof. Karsten, of Berlin, placed a coin 

 upon glass, and by electrifying it made a latent impres- 

 sion, which revealed itself when breathed upon. About the 

 same time Mr. W. R. (now Sir W. R,) Grove made similar 

 impressions with simple paper devices, and fixed them so as 

 to be always visible. A discussion of Karsten's results occurs 

 in several places, but I have not been able to find details of 

 his method of performing the experiment. During my 

 attempts to repeat it some effects have appeared which seem 

 to be new and worthy of record. 



After many trials I found the following method the most 

 successful: — A glass plate, 6 inches square, is put on the table 

 for insulation : in the middle lies a coin with a strip of tinfoil 

 going from it to the edge of the glass : on this coin lies the 

 glass to be impressed, 4 or 5 inches square, and above it a 

 second coin. It is essential to polish the glass scrupulously 



* Communicated by the Physical Society : read June 24, 1892. 



