260 Mr. MacKain on the Compression of Water. 



ing its compressibility, are composed, have enabled philosophers to 

 determine with considerable accuracy the ratio wliich obtains be- 

 tween the force applied and the resulting condensation of volume. 



A considerable time ago it was believed that the compressibility of 

 air was in proportion to the pressure applied ; this was subsequently 

 proved nearly 200 years ago, by Boyle, and also by Mariotte about 50 

 years afterwards ; and this law of compression has since been known by 

 the name of the latter. More recently Messrs. Dulong and Arago have 

 confirmed the accuracy of the law of Mariotte, by experiments con- 

 ducted to the range of no less than 27 atmospheres beyond the common 

 atmospheric pressure. 



By means of the barometer, the density of air is found to vary ac- 

 cording to its mass superincumbent over any given point in the atmos- 

 phere, and the numerous experiments made with this instrument, have 

 brought to such a degree of accuracy the barometrical measurements 

 of parts of the earth's surface protruding into the air, as to vie with 

 measurements of their heights made by trigonometrical instruments. 

 These degrees of density are measured by a column of mercury, and 

 consequently, the height of the column indicates the force of compres- 

 sion, and represents the height of the superincumbent mass of air. 



The extent of compression which water undergoes, when subjected 

 to force, has engaged the attention of men of science for some time 

 back. In 1762, Mr. Canton found that the addition to, or subtraction 

 from water, of a weight equivalent to that of the atmosphere, produced 

 at the temperature of 60° a contraction or extension of rain water of 

 4 millionth parts of its bulk, and of sea water of 40 millionth parts, 

 while in mercury it only amounted to 3 millionth parts : showing that 

 the density of the fluid operated on materially affected the results. 

 Thus, in the case of rain water, a force equal to a column of itself 33^ 

 feet in height produced a contraction of 46 millionth parts : of sea water, 

 a column 32 feet in height produced a contraction of 40 millionth 

 parts : and in mercury, a corresponding column of 2i feet produced 

 3 millionth parts of compression. Zimmerman, Professor at Bruns- 

 wick, Professor (Ersted, of Copenhagen, the late Sir John Leslie, of 

 Edinburgh, and Mr. Perkins, have made numerous experiments that 

 establish the fact of compression ascertained by Mr. Canton, which, 

 at the time his experiments were published, was at variance with the 

 opinions universally entertained on this subject. With the usual 

 haste which Sir John Leslie speculated on experimental results, he 

 arrived at the conclusion " that the ocean may rest on a subaqueous 

 bed of air," from the apparently greater degree of condensation 

 which force can produce in air, in contrast with that which similar 

 forces were supposed capable of producing on water. 



The degree of compression of water, is, however, extremely small ; 

 and the force which it is necessary to apply to it, in order to produce 

 any appreciable degree of diminution in volume, is so great in proper- 



