204 ON THE PHYSICAL STRUCTURE OF THE EARTH. 



pared with solids. I have before now referred to the statement of the 

 same proposition in the comprehensive work of the late Prof. O. F. 

 Naumann, the Lehrhuch der Geognosle, vol. i, p. 2G9, 2d 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 com- 

 pressible 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 iucorai)ressible. 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 100 miles it would be 

 scarcely correct to make such an assumption. The compressibility is 

 negligible in a small mass of water, but it can not be neglected in a 

 large mass. Such an assumption is equally unwarrantable with regard 

 to properties of matter which, though negligible in some problems, are 

 not in others. Thus in the common hydraulic questions liquids are 

 assumed to be incompressible ; it would be more correct to say the com- 

 pressibility is neglected. In small problems connected with limited 

 portions of the atmosphere the comi)ressibility of air may be also neg- 

 lected, but we could not neglect it for a high column of the atmosphere. 

 If, as before remarked, the Earth were surrounded with an ocean 100 

 miles deep, the compressibility of the water could not be well over- 

 looked in tidal questions; then, « /or^ior/, compressibility can not be 

 neglected in such a problem as the tides of a liquid spheroid having a 

 radius nearly equal to that of the earth. This is immediately made 

 manifest by expressing the compressibilities of liquids, not in terms of 

 the amount due to a single atmosphere of pressure, as is done in most 

 tabulated groups of results, but by some very much greater standard, 

 such as one or two thousand atmospheres. In the experiments of Per- 

 kins t the highest pressure employed was 2,000 atmospheres, and with 

 this he reduced a column of water by nearly one-twelfth of its volume. 

 Tbe results of experiments with great pressures such as this are highly 

 illustrative of the force by which a fluid may be compressed in the 

 Earth's interior. The actual coefficients of cubical com])ressibility, on 

 which calculations could be based, may be partly obtained from the 

 more exact researches of Kegnault, Grassi, and other recent experi- 

 ments, or from special investigations on fluid matter conducted with 

 precautious such as these observers have employed. By then compar- 

 ing the moduli of compressibilities calculated from pressures of 1,000 

 or 10,000 atmospheres there could be no possibility of overlooking the 

 consequences as to the relations of liquids and solid bodies in any case 

 where they could be subjected to pressures of abnormal magnitude. 



(3) The propagation of sound in liquids and solids gives further proof 

 of the greater compressibility of liquids. 



* "Fliissige Korpersind aber mit einer weit starkeren Compressibilitat begabt, also 

 starre Ivorper. " 



t Phil. Trans. 1826, p. 541. 



