236 Prof. H. Hennessy on the 



facts were not clearly put forward, I underrated the com- 

 pressibility of liquids as compared with solids. The influence 

 of the imperfect experiments of the Academia del Cimento 

 has long injuriously operated in defining liquid and solid 

 matter, and has produced a remarkable conflict of opinions. 



On taking the results of the best experimental investiga- 

 tions, it appears that, although liquids are but slightly com- 

 pressible as compared with gases, they are highly compres- 

 sible as compared with solids. In many treatises on Physics 

 and Mechanics which have a high reputation matter is divided 

 into solids, elastic fluids or gases, and incompressible fluids 

 or liquids. Hence the erroneous inference seems to have 

 arisen that liquids are incompressible, not only in comparison 

 with gases, but also in comparison with solid bodies. I was 

 surprised to find this remarkably misleading proposition for- 

 mally stated long after the decisive experiments of Oersted, 

 Colladon and Sturm, Regnault, Wertheim, and Grassi, in such 

 a work as Pouillet's Elements de Physique, and also in the 

 German translation by Miiller. The greater compressibility 

 of liquids as compared with solids is seldom affirmed as a dis- 

 tinct general proposition in books on Physics. It occurs, 

 however, in Deschanel's treatise, both in the original and in 

 the English edition. Daguin states, in vol. i. of his Traite 

 de Physique, 2nd edition, p. 40, that the compressibility of 

 liquids was long considered doubtful, but nevertheless they 

 are more compressible than solids. 



Lame also pointed out the great compressibility of liquids 

 as compared to solids. I have before now referred to the 

 statement of the same proposition in the comprehensive work 

 of the late Professor C. F. Naumann, the Lehrbuch der 

 Geognosie, vol. i. p. 269, 2nd edition*. 



Although in many physical questions the compressibility 

 of liquids may be neglected as well as the compressibility 

 of solids, we are not entitled to assume at any time that the 

 latter are relatively more compressible than the former. In 

 questions where the pressure of columns of liquid of great 

 magnitude comes under consideration, we can no longer treat 

 the liquid as incompressible. In the problem of oceanic tides 

 the incompressibility of the water has been assumed, but if a 

 planet were covered with water to a depth of one hundred 

 miles it would be scarcely correct to make such an assump- 

 tion. The compressibility is negligible in a small mass of 

 water, but it cannot be neglected in a large mass. Such an 

 assumption is equally unwarrantable with regard to properties 



* " Fliissige Korper sind aber mit einer weit starkeren Coinpressibilitat 

 begabt, als starre Korper." 



