rtr 



PRESSURE, CENTRE OP. 



I'Ui: 



against the Kale, or that weight hung to a beam pull* the 



Hiring said thus much, nothing remaint as to the manner of esti- 

 mating pressures, except to give a reference to the articleB KOHCI:, 

 Jl.iM, MoMtsTt-M, MOTION (LAWS or), *o. 



\\ tltcnce collect that in problems of dynamic* it U requisite to 

 wtimate pressure (in rinni in which it cannot l>o balanced against a 

 weight) by the velocity which it would produce or destroy in a given 

 OHM of matter, and in a given time : it being known that the weight 

 of a mir means that pressure which, applied to the man, would pro- 

 duce in one second Si-l'.i fvt of velocity. An instance of this process 

 oocun in CKXTRIPITAL AND CKXTKIH -HAI. KOKCES. 



Pressure can only produce an effect in time an instantaneous 

 alteration of velocity (as noted in luri'LSK) being mechanically absurd. 

 But if a pressure produce its whole effect in an imperceptible time, 

 there is all the ap|>earance of an instantaneous destruction or creation 

 of velocity. S<i|i]>o*e a hammer, for instance, to strike on anvil with a 

 rdocity of 20 feet in a second. The instant the contact begins, both 

 hammer ami anvil begin to be compressed, and the compression begins 

 at the rate of 20 feet a second. But the resistance to compression is 

 enormous, and is a pressure which, though it takes time to destroy 

 any velocity, yet will destroy a velocity of 20 feet a second in a very 

 mall fraction of a second. The moment the velocity is all destroyed 

 the effort of the anvil and hammer (both of which are compressed) 

 endeavouring to restore themselves, the continuation in fact of the 

 pressure which destroyed the velocity, will give a velocity to the ham- 

 mer in a contrary direction, or the hammer will rebound, as it is well 

 known to !. In the appendix to Professor Whewell's ' Elementary 

 Treatise on Mechanics' (third or fourth edition), a mathematical inves- 

 tigation of such problems (by Mr. Airy) will be found, upon highly 

 probable hypotheses as to the constitution of matter. The following 

 remit of these hypotheses (which cannot bo far wrong) will give an 

 idea of the enormous pressures which are created in common cases of 

 what is called impact : If a column of iron three inches high be let 

 fall on an anvil two feet high, from a height of eight feet, the com- 

 pression of both hammer and anvil is about one-thousandth of a foot, 

 and the pressure exerted at the moment when it is greatest is that of a 

 quiescent column of iron similar to the hammer in its action, but of 

 no leas than 4000 feet high. 



PRESSURE, CENTRE OF. When a fluid presses upon a surface, 

 there is a point in that surface (known as the centre of preuure) at 

 which, if a force be applied in the same line with the pressure of the 

 fluid and equal to the whole of that pressure, but in a contrary direc- 

 tion, it will balance or counteract the whole pressure of the fluid. It 

 is often of great importance to find the centre of pressure, and for the 

 methods of doing so we must refer to HTDBOST AT i 



PRESUMPTION. A presumption may be denned to be a belief or 

 inference as to the existence of a fact not actually known, arising from 

 its necessary or usual connection with others which are known. 



A fact may be proved by the immediate knowledge of the witnesses 

 to it, which is called direct evidence. If it cannot be so proved, some 

 other fact may generally be proved by direct evidence, from which the 

 fact in question may often be inferred. If such other fact c.-m 1 >. 

 proved, and the existence of the fact in question can be inferred, such 

 inference is a presumption. The inference may be either strictly 

 logical or necessary, or it may be only probable, that is, the fact 

 inferred may be true or it may not be true. If we cannot infer from 

 the fact proved that the fact in question may be true, there can be no 

 presumption at all as to such last fact. In all cases then, in order to 

 establish a presumption, there must necessarily be an inference from a 

 fact or facts ; but the inference may be either necessary or probable. 

 If necessary, it cannot, by the supposition, be disproved ; if probable, 

 it may cither be disproved by evidence, or it may not be possible to 

 j it for want of evidence, and yet the inference will still only be 



Presumptions which are necessary can hardly ever be considered as 

 otherwise than conclusive in any system of law. Presumptions which 

 are probable only may, by positive law, be made as conclusive as 

 necessary presumptions, that is, it may not be permitted to disprove 

 them when they could be disproved; or where such disproving 

 evidence is wanted, and yet the inference is only probable, positive law 

 may give it the same conclusive force as a necessary presumption. 



A presumption, when established, that is, a fact when presumed, is 

 legally the same u a fact proved in such manner an the particular 

 system of law requires such fact to be proved. If then the law annexe* 

 any legal consequence* to a given fact when ).r..v. ,]. it annexes the 

 mine to it when the fact is legally presumed. It is only by virtue of 

 legal consequences being annexed to facts that they become objects of 

 jurisprudence. The establishment then of a presumption, in a legal 

 *, U only the establishment of a fact to which certain legal con- 



: . . - . .... 



In our o 



ir own system, the presumption is sometimes made by a judge 

 or a number of judges, and sometime* by a jury, but the consequences 

 are the same. Some writers say that presumptions are cither " legal 

 and artificial" or "natural." Thi-y divide "artificial or legal pre- 

 Hiimptions " into two kind*, immediate and mediate. " Immediate are 

 those which are made by the law itself directly and without the aid of 

 a jury. Mediate presumptions are those which cannot be made but 



by the aid of a jury." Presumptions may therefore be divided int > 

 three classes: 1, Legal presumptions made by the law itself, or 

 presumptions of law; 2, Legal presumptions to be made by a jury. <>r 

 presumption* of law and fact; 8, Mere natural presumptions, or pre- 

 sumptions of fact." 



The first class of presumptions, it is said, are either absolute and 

 conclusive, or they may be rebutted by evidence to the contrary. The 

 presumption of law that a bond was executed upon a good considera- 

 tion cannot I* rebutted by evidence, so long an the I., .u.l i- nun Matched, 

 that is, so long as it is admitted to be a bond. But though the law 

 presumes that a bill of exchange was accepted on good c- 

 it admits evidence to show that such was not the fact. Now this 

 presumption of law is nothing more than a fact presumed by a jud^e 

 or judges, to which fact so presumed, certain legal consequences are 

 annexed or belong. It is however a very inaccurate expression to 

 speak of a presumption of law ; for, " when the law presumes or 

 any fact to which a legal consequence is annexed from any >! 

 predicament of facts, the law in effect indirectly annexes to that 

 predicament the legal consequence which belongs to the pr< 

 fact" 



One presumption of law may be opposed by another, and the law, 

 that is, the court, must then decide which is the stronger. 



Presumptions of mm laa, as shown, are such as are made by the 

 court. There are instances of presumptions made by act of parliament. 

 that is, the legislature has declared that a certain fact or facts, when 

 proved, shall be conclusive proof of another unproved fact wliieh is 

 not a necessary, and, it may be, is often not a highly probable inference 

 from the proved fact. A statute of 21 James I., c. 27 (now repealed), 

 made proof of the concealment of the death of a bastard child by the 

 mother conclusive evidence of her having murdered it, unless she 

 conlil prove that it was bom dead. And sometimes an act of 

 parliament declares that a certain presumption shall not be allowed or 

 made. (2 and 3 Win. IV., c. 71, s. 6.) A presumption of mere law 

 is sometimes called an intcni/ment of law. 



Presumptions of laic and fact ore artificial presumptions, recognised 

 .end warranted by the law as the proper inferences to be made by 

 juries under particular circumstances. In other words, these are facts 

 which the law, that is, the court, will allow a jury to presume from 

 other facts proved by direct evidence. When the presumed fact is 

 declared by the jury to be a real fact, or ia implicitly contained in 

 their verdict, the legal effect is the same. It is said that the inference 

 (made by the jury) is never conclusive, which appears to mean that 

 there are presumptions which ore not necessary, and sometimes may 

 not be highlv probable, but they are still such as a jury may make (at 

 least under tne direction and advice of the court), and their verdict 

 will be good. Thus a jury is required, or at least advised by a court 

 to infer a grant of an incorporeal hereditament after an a 

 enjoyment for the space of twenty years. The presumption of right 

 in such cases is not however conclusive; in other words, it is not 

 an inference of mere law to be made by the courts, yet it is an 

 inference which the courts advise juries to make whenever the pre- 

 sumption stands unrebutted by contrary evidence. Such evidence 

 in theory ia mere presumptive evidence; in practice and effect it 

 ia a bar. 



The third class, the [natural presumptions of mere fact, are wholly 

 independent of any artificial legal relations and connections, and ilill'cr 

 from presumptions of mere law in this essential respect, that those 

 depend upon or rather are a branch of the particular system of 

 jurisprudence to which they belong; but mere natural presumption* 

 are derived wholly by means of the common experience of mankind 

 from the course of nature and the ordinary habits of society. This 

 class of presumptions properly belongs to a jury, and yet the < 

 will sometimes make presumptions of this kind without the aid of a 

 jury. These presumptions then ore such as a jury may make without 

 the advice or direction of the court, and it seems therefore to be a 

 general rule, that whenever there ia evidence on which a jury have 

 founded a presumption according to the justice of the case, the court* 

 will not grant a new trial. 



Presumption then is either a positive rule by which a certain con- 

 clusion is declared by statute, or by the judges, or by the jury 

 under the direction and advice of' the judges, to follow from certain 

 other proved facts; or it is a conclusion from certain other proved 

 facts which a judge or a jury may make if they find the probative force 

 of the proved facts sufficient to induce them to moke tne inference 

 called a natural presumption, or presumption of mere fact. Presump- 

 tions therefore are incident to every head of law in which proof is 

 required ; and the presumptions which are positive rules of law are 

 part of the law of the things to which they relate. 



The term " pncsumptio occurs occasionally in the ' Digest,' and in 

 the sense of an inference from a fact^roved or admitted. (' Dig.', 22, 

 tit. 3. s. 25.) 



(Bentham, Rationale of Judicial Evidence ; Starkie, On Evidence ; 

 Phillip.*. OH Entrnee.) 



PRESUMPTIVE HEIR. [DESCENT.] 



PRICK. [VALI-K; WAGES.] 



PU1KST (Saxon, preott, from the Greek vparfturtpot, an elder), in 

 its popular acceptation, ia the name of a minister of religion in all 

 ages and countries. 



