396 



COMPRESSION 



COMRIE 



compressed air has also been found to be the mos 

 convenient power (see BRAKES). Air compresse 

 and stored in a reservoir under the vehicle has als 

 been proposed as a motive power for tramway cars 

 In .a different direction the agency of compressec 

 air is important in the artificial production o 

 cold for chilling-houses for meat-preservation on 

 land, and for frozen-meat chambers for preserving 

 fresh meat on board vessels (see REFRIGERATION). 

 Compression and Compressibility. When 

 a body is subjected to the action of any force whicl 

 causes it to occupy less volume, it is said to be 

 compressed, and the diminution of volume is termec 

 compression. The term compressibility is fre 

 quently used to signify that property of bodies 

 whereby they yield to that particular form of stress 

 known as pressure ; but more strictly it is employee 

 to denote the measure of this property as possessee 

 by different substances. Under the same pressure 

 it is obvious that the same volume of various 

 substances will diminish by different amounts 

 and, to measure this change, the compressibility 

 is denned to be the ratio of the amount ol 

 compression per unit volume to the compressing 

 force applied. It thus may be determined by 

 measuring the amount of compression of a known 

 volume when under a certain pressure ; dividing 

 this by the product of the original volume and the 

 pressure gives the average compressibility (per 

 unit pressure) of the substance throughout the 

 range of pressure employed. The unit of pressure 

 generally used is one atmosphere, which is denned 

 in this country as being the weight of a column of 

 mercury, one square inch in section, 29-905 inches 

 in height, at the temperature of C., and weighed 

 at sea-level in the latitude of London. Its actual 

 value in pounds-weight per square inch is nearly 

 14'7 ; so that 152-3 atmospheres of pressure is 

 equivalent to a pressure of one ton per square inch. 

 In gases the relation between pressure and volume 

 is given by Boyle's Law (see GASES) viz. the 

 volume of a given mass of gas is inversely pro- 

 portional to its pressure. From this it follows that 

 the compressibility is inversely proportional to the 

 pressure i.e. the diminution of volume due to a 

 given increment of pressure is correspondingly 

 small as the pressure is great. The behaviour of 

 a gas under pressure is closely related to the 

 proximity of its temperature to the critical point 

 (see CRITICAL TEMPERATURE); for if below this 

 temperature the gas can, and if above 

 it, cannot be liquefied by pressure 

 alone. It is only since 1877 that 

 liquefaction has been effected in those 

 gases formerly termed permanent. 



From the first attempts to compress 

 liquids it was concluded that they 

 were incompressible, but Canton in 

 1762, by a comparatively simple experi- 

 ment, showed that the compressibility 

 of water though small is quite appre- 

 ciable, and that it is less at higher 

 than at lower temperatures. The 

 measurement of the compressibility of 

 liquids is usually made in a glass vessel 

 ( see fig. ) termed a piezometer. A tiibe, 

 ABCD, open at one end, D, is bent 

 upon itself between C and D, widened 

 at one end into a cylindrical bulb, AB, 

 and at the other into a cistern, D. The 

 liquid experimented on fills the bulb 

 and stem to C, from which point to 

 D, mercury fills the tube. On the sur- 

 face of the mercury at C an index 

 floats. The instrument is placed in a larger and 

 much stronger vessel containing water to which 

 pressure (measured by an attached gauge) is 

 applied. The contents of the piezometer being 



i 



thus compressed, the mercury column ascends in 

 the stem, and when the pressure is relieved the 

 index is left at that point to which the mercury 

 rose under the highest pressure applied. The 

 actual amount of compression, and the original 

 volume, as well as the pressure, being known, the 

 compressibility can be thereby calculated, a correc- 

 tion being finally added for the compression of the 

 glass piezometer itself. From experiments made 

 with such apparatus, the following conclusions ( see 

 Report on some of the Physical Properties of Fresh 

 Water and Sea-water, by Professor P. G. Tait ; 

 Challenger Expedition Commission Reports, Physics 

 and Chemistry, part iv. ) seem now to be well estab- 

 lished regarding the compressibility of liquids, more 

 especially of water. The compressibility of water 

 decreases as both the temperature and pressure are 

 raised; under moderate pressures (e.g. one or two 

 atmospheres) it has a point of minimum value about 

 60 C., while its actual value at 10 C. and at a 

 pressure of one ton per square inch is very nearly 

 sTf^ir- Sea-water is less compressible than fresh 

 water ; the ratio of the compressibility of the 

 former to the latter being -915. Solutions of com- 

 mon salt are less compressible as they are stronger ; 

 the compressibility falling off uniformly with in- 

 creased strength. Both sea- water and salt solutions 

 diminish in compressibility with temperature and 



Eressure in the same manner as fresn water. It 

 as also been proved that the maximum-density 

 point of water is lowered by pressure ; the actual 

 amount of this lowering being 3'l C. per ton 

 i.e. water under a pressure of one ton per 

 sq. in. has its maximum density point at 0-9, 

 instead of at 4, as under ordinary atmospheric 

 pressure. 



The compressibility of solids is generally very 

 much smaller than that of either liquids or gases. 

 It is best measured by noting the shortening 

 of a rod or fibre of the material tested while 

 subjected to hydrostatic pressure ; the linear com- 

 pressibility thus obtained is, to a sufficient degree 

 of approximation, one-third the cubical compres- 

 sibility. For glass it is -00000265 per atmo- 

 sphere. 



Compulsion. The effect of compulsion on the 

 validity of obligations and payments, and on 

 riminal responsibility, is noticed under CONTRACT, 

 }RIME, FORCE AND FEAR, and DURESS. 



Coinplirgators were twelve persons whom 

 Anglo-Saxon law permitted the accused to call in 

 jroof of his innocency, and who joined their oaths 

 ;o his. They were persons taken from the neigh- 

 xmrhood, or otherwise known to the accused. It 

 was rather in the character of witnesses than of 

 urymen that they acted, though the institution ha 

 jeen spoken of as the Anglo-Saxon jury ; what 

 ,hey swore to was not so much their knowledge, as 

 jheir belief. The number of compurgators varied 

 vith the rank of the parties and the nature of the 

 iccusation, but was usually twelve. The system 

 )f compurgators was adopted even in civil actions 

 or debt. Compurgation, which was a custom 

 :ommon to most of the Teutonic races, fell into 

 isuse after the conquest ; but the ceremony of 

 vhat was called canonical purgation of clerks-con- 

 vict, was not abolished in England till the reign of 

 Elizabeth. See (under Jury) JURY TRIAL. 



Comrie, a pleasant and sheltered village of 

 'erthshire, on the Earn, 7 miles W. of Crieff. It 

 as often been visited by earthquakes, notably in 

 he October of 1839 and January of 1876. These 

 re apparently due to its geological position on the 

 reat line of fault between the Highlands and the 

 iOwlands. Here George Gilfillan was born in 1813. 

 ?he Free church, built in 1879-81, cost over 10,000. 

 ['he railway hither was opened in 1893. Pop. 870. 



