465 



DEMONSTRATION. 



DEMULCENTS. 



406 



the Spanish lines before Gibraltar in 1810, several attempts were made 

 to run a gallery from the interior of the work to the back of the 

 revetment, but the sand so constantly came in at the head of the 

 gallery, that it at last brought down the terreplein, and eventually 

 shafts had to be sunk at the back of the revetment, which is in 

 general the commonest and easiest method of execution. When the 

 shaft or gallery has been driven to the foot of the revetment, short 

 returns are usually made to the right and left along the back of the 

 revetment, and the charges either placed against or in the counterforts, 

 or disregarding them, as at the destruction of the face of a bastion at 

 Turin, which was successful at equal distances along the revetment. 

 Gen. Sir Charles Pasley, in his tract on 'Military Mines,' gives 3 g LLR 3 

 [MIXES], as the charge when placed at 2-lined intervals along the 

 back of the revetment without counterforts, i LLR", when in the 

 middle of each counterfort at its junction with the revetment, and 

 .'. LLR 3 when in the centre of a line of masonry with counterforts at 

 2-line intervals, whilst fa LLR 1 is the charge when placed in the centre 

 of a mass of masonry of a circular or polygonal base. 



Towers are usually destroyed either by mining the base, as with 

 revetments, or by placing the charge in the centre of basement on the 

 floor, or distributing at the angles of the basement. 



Magazines, if time will permit, are best destroyed by a series of 

 mines placed in the centre of the thickness of ita piers and of its gable ; 

 but they may be hastily destroyed by placing the powder in a heap on 

 the floor, fastening the doors and windows, and firing by a hose from 

 the outside. 



Buildings may be destroyed by charges placed in the same way in 

 the cellars or lower rooms, or by placing a series of small charges or 

 blasts in the thickness of the wall of the basement between the 

 windows, the holes being driven from the windows. 



Various methods were adopted in the Peninsula war for destroying 

 bridges, according to circumstances. It frequently happens whilst 

 an army is before the enemy, and closely pressed by him, that a bridge is 

 required to be destroyed to prevent his pursuing a retreating body, or 

 to gain time for the movements of an army by arresting his progress. 

 Sir H. Jones says in the article before mentioned, " In the retreat of 

 the British army from Burgos, upwards of twenty bridges were de- 

 stroyed, with the exception of two or three, which were only partially 

 injured from want of tune ; the destruction of the others was perfect ; 

 in many cases the train was not lighted until the enemy were actually 

 on the bridge ; hi others, as at Cabezon, the enemy's pickets were two 

 or three days at one end of the bridge ; an officer of engineers, during 

 that time, waiting with a lighted slow match, prepared to fire the 

 train the instant the enemy should attempt to push on to the bridge ; 

 at the same time cautioned not to be deceived by false alarm of his 

 advance. In many instances an officer of engineers was unexpectedly 

 called upon for the duty of destroying a bridge, sent off at a moment's 

 notice forty or fifty miles, to be followed by a muleteer carrying two 

 barn-Is of ]>owder ; without a miner or tools, other than what could 

 be collected in the neighbourhood of the bridge to be destroyed, and 

 with such assistance in manual labour as could be procured on the 

 sput, or by the assistance of some of the dragoons forming his escort." 

 The fate of the campaign may depend on the proper execution of 

 tlii.-i duty. The following modes are usually adopted : 1. By sinking a 

 shaft in the roadway, generally a few feet to the right or left of the 

 centre of the width of the bridge, down to the haunch of the arch, 

 with a very short gallery ending in a chamber, so as to lodge the 

 powder in the middle of the width of the bridge under the roadway. 

 If the charge is to remain for some time, care must be taken to secure it 

 and the hose or train from damp and injury by traffic over the bridge. 

 2. When the bridge is very wide, by sinking two shafts and placing 

 two charges of powder though this method is liable to failure by one 

 charge only exploding, and leaving half the bridge standing. 3. Lay- 

 ing a quantity of powder on the crown of the arch, so as to break it 

 d'irt-ii by the concussion ; the French say 100 Ibs. are sufficient for this ; 

 iwever doubtful. 4. Suspending the charge under the crown of 

 the arch. 6. Driving a gallery into the pier at the springing of the 

 arch, which would be a good plan but for the difficulties attending ita 

 execution. " At Duenaa in 1812, on the retreat from Burgos, the rear 

 guard of the army was closely pressed by the enemy ; the bridge was 

 of solid masonry from the arch to the roadway ; the miners had only 

 time to strip off some of the pavement, and lodge two barrels of 

 powder in the hole, covering them as hastily as possible with the small 

 quantity of material at hand : when fired, the effect was to break down 

 the entire breadth of the arch, making a gap of 15 feet." Barriers were 

 formerly destroyed by the petard, this, however, is a clumsy instru- 

 to carry about, and at the present day a bag of powder of about 

 60 to 1 00 Ibs. is generally all that is found necessary. If suspended 

 or laid against the barrier or gate ita explosion is generally found to 

 be effectual, as at the late siege of Delhi, and at Ghuznee in 1839, 

 where, however, 300 Ibs. in twelve sand-bags was used. 



DCKOK8TRATION. The old use of this word is as follows : It 

 t any manner of skoiriny either the connection of a conclusion 

 with ita premises, or that of a phenomenon with the asserted cause. 

 Thus we find it asserted by an early writer that the system of Co- 

 pernicus, t>inni//i falte, gave truer demonstrations of the celestial motions 

 than any other : meaning that those motions could be more easily 

 shown to be a necessary consequence of the hypothesis cited than of 



ABTS AND SCI. DIV. VOL. III. 



any other. It is very important, in reading the old English writers, 

 to remember this use of the term. 



Demonstration now means only that process by which a result is shown 

 to be a necessary consequence of the premises from which it is asserted 

 to follow, on the supposition that those premises are admitted, either 

 as matter of fact, or of intuitive evidence, or of previous demonstra- 

 tion, Thus the 47th proposition of the first book of Euclid demon- 

 strates a certain property of a right-angled triangle, on the supposition 

 1, that all the preceding propositions are true ; 2, that the axioms 

 used in geometry, whether expressed or implied, are true also. It 

 makes the consequence as certain as the premises, by means of the 

 indubitable character of the connecting process. This strict use of the 

 term demonstration belongs to the science of logic, which contains the 

 art of demonstrating from premises, without reference to the truth or 

 falsehood of the premises themselves. 



But in common life the word demonstration means any sort of 

 reasoning which the party using the term chooses to call proof. " I 

 consider it as demonstrated," means the same thing as " evidence has 

 been offered which makes it so probable to me that I am willing to act 

 upon that evidence." Most minds have no idea of a middle state; 

 they either absolutely receive or absolutely reject : so that in fact 

 demonstration comes with any degree of balance of evidence on one 

 side or the other. It is easy to admit that in common life high pro- 

 bability is the ground of assent, and that it never can be otherwise : 

 but there is mischief in the use of the same term both for logical 

 inference and for its mixture with matters of guess, or feeling, or 

 unassisted perception. The logician might consent to abandon the 

 word demonstration for popular use, and to adopt another : but it 

 would result that the new word, whatever it might be, would imme- 

 diately be seized by those who confound what is commonly called 

 argument with what is actually demonstration. Whatever may be 

 the strongest term in use, it will be adopted by strong asserters : nor 

 will he who has shown probable reason, however great, for the adop- 

 tion of a result of argument, be satisfied unless his conclusion be 

 received as certain. What we have here said refers entirely to the use 

 of the word demonstration : the fallacy to be feared is the placing in 

 the result of it, in one sense, the degree of confidence which ia only 

 obtained by confounding that sense with another. Two parties, both 

 convinced that a conclusion has a high degree of evidence, dispute 

 whether the evidence amounts to demonstration. To settle this point, 

 they sift the evidence most minutely, and perhaps entirely forget to 

 inquire whether they mean the same thing by the word demonstration. 

 Every proposition has its contradiction : and of these one must be 

 true and one false. When the truth of the proposition is proved at 

 once, the demonstration is called direct : but when the contradiction 

 is proved to be false, the demonstration is called indirect. (ABSUBDUM, 

 REDUCTIO AD.) 



DEMONSTRATIONS, in military affairs, are operations of any 

 kind which may be performed for the purpose of deceiving the 

 enemy respecting the measures which it is intended to employ 

 against him. 



They consist in displaying an apparent activity in forming or re- 

 pairing a road, or in sending provisions or stores to a particular 

 place, as if preparatory to a march of troops in that direction; in 

 marking out ground as if for an encampment ; and in detaching bodies 

 of troops to make false attacks. Such demonstrations are made chiefly 

 with a view of inducing an enemy to divide his forces, and thus 

 weaken his line at points against which the real movement or attack 

 is intended to be directed. 



In order to succeed, however, in these demonstrations, they should 

 be made with great precaution, that they may have every appearance 

 of indicating a serious enterprise : even the troops who are to exe- 

 cute them should be unacquainted with the object in view, lest a 

 deficiency of energy should betray the secret to the enemy, who must 

 be supposed to be on his guard against such attempts, and who may 

 consequently take measures to render them useless. 



DEMULCENTS are medicinal agents which have the property of 

 protecting sensible surfaces from the action of irritating matter, by 

 hindering it from coming in direct contact with them. They are 

 thus distinguished from diluents, of which water may be taken as an 

 example, which often lessen acrimony by diluting or attenuating the 

 fluid in which it exists. When much water is present in any demul- 

 cent liquid, the action is partly that of a diluent, but the chief benefit 

 results from the bland nature of the substance, or from its viscidity. 

 Demulcents are either solutions, such as mucilage of gum arable, or 

 certain substances mechanically diffused through water or milk, such 

 as wax, spermaceti, or suet. Their beneficial effects are greatest over 

 the mucous surfaces with which they come into immediate contact, 

 such as the throat, stomach, and intestinal canal, but by sympathy 

 their soothing action would appear to be extended to the mucous 

 surfaces of the lungs, and of the urino-genital organs. Mucilaginous 

 substances such as gum, are not easily digested, unless some bitter 

 or astringent principle be taken along with them ; so that demulcents 

 pass along the intestinal canal, shielding it from irritating substances 

 throughout its entire length, though less effectually as they proceed, 

 and become mingled with the various secretions in their passage. 

 They thus afford considerable protection to the inner coat of the 

 stomach and intestinal canal against poisonous agents, and for this 



H u 



