Prof. P. Tardy on a New Equation in Hydrodynamics. 171 



tension, the passage of a jet of water properly guarded so as 

 to give no electrical indications would increase the tension ; in 

 other words, would the condensation or running together of 

 the vapour particles cause an increase of tension in the way 

 suggested by the Committee of Physics of the Royal Society? 



Mr. Phillips's experiment, that bears more particularly on 

 the point in question, is the mixture of steam and water in a 

 tin pipe. When the steam was introduced alone, no electrical 

 effects were exhibited ; and when the water was introduced 

 alone, there were also no electrical effects ; but when they 

 were mixed, a positive charge was indicated by the electro- 

 meter. The experiment was thus varied : on one occasion 

 the steam was first introduced ; and upon the jet of water 

 being projected among the particles of steam, the mixture 

 was electrified positively : on the other occasion the water was 

 first introduced ; and upon the jet of steam being projected 

 into the tin pipe, the mixture was also electrified positively. 

 These two experiments appear to me to be conclusive on the 

 subject of the development of the electric force by condensa- 

 tion alone, and become a link in the chain of evidence by 

 which nearly all the varied, beautiful, grand and terrific phae- 

 nomena of the atmosphere are bound together. This one 

 simple but efficient mode of the development of the electric 

 force appears to be fully capable of accounting, if not for the 

 whole, for the greater portion of the phaenomena of atmo- 

 spheric electricity. 



Kew Observatory, Feb. 6, 1850. 



XXIII. Some Observations on a New Equation in Hydrody- 

 namics, ^j/ Professor P. Tardy of Messina*. 



I HAVE lately seen an article by Professor Challis in the 

 Number of the Philosophical Magazine for last June 

 (Supplement), "On certain points relating to the Theory of 

 Fluid Motion," in which, in reply to a communication of M. 

 Bertrand to the French Academy of Sciences, he returns to 

 his favourite argument for the truth and necessity of a new 

 equation in hydrodynamics. At the same time he does me 

 the honour of mentioning my name, and answering an objec- 

 tion which I made to Jiis analysis for the case of the motion in 

 space of two dimensions. 



I must premise that when I wrote my memoir on the move- 

 ment of fluids {Sopra alcuni Punti delta Teoria del Moto de' 

 Liquidij Fi renze 1 847), I had no knowledge of Professor Challis's 



• Communicated by the Author. 



