POWER OF ATTORN F.Y. 



PRACTICE. 



which i n-wchanically equivalent to twenty pounds moved through one 

 foot, uid UM paradox disappear*. It u usual to rxprras this by (tying 

 that' hat w (rained in power i lost in time ; but instead of this axiom , 

 which only indirectly expra*M th.- truth, it in I..-U.T u. ny that a 

 less weight cannot cam* the (scent of a greater through a given height, 

 without deer-finding through a height greater in the tame proportion a* 

 that by which the greater weight exceed* the lew. Perhaps the 

 naiJTT* mode of h'"?ir)g the power applied and the work done is aa 

 follow* : Suppose one pound descending through four feet mine* four 

 pound* through one foot, we have then applied the descent of one 

 pound through one foot four time*, and produced the accent of one 

 pound through one foot four time*. The only difference U, that in 

 the applied power it i* the aame pound which move* over four different 

 feet, while in the produced effect there are four different pound* 

 moving over the aame foot. The machine i* nothing but an adapta- 

 tion which enable* the agent to effect the exchange just mentioned. 

 And though the common notion that a machine give* power usually 

 ha* reference to the mbntittitinn'of a greater weight with lee* velocity 

 for a smaller weight with greater velocity, yet it frequently happen* 

 that the apparent power in gained by the contrary exchange, ax in Un- 

 common grinding-wheel and in the lathe. Frequently al*o the benefi- 

 cial effect arise* from a reservoir of power which i* given out in i-mall 

 quantitie*, a* in a clock or watch, in which the exertion of raising a 

 weight or coiling a spring i* expended in minute quantities over one 

 or more day*. 



Without entering further into the preceding principle (for which, in 

 it* mathematical bearing, see VIRTUAL VELOCITIES), we will now 

 touch upon some additional circumstances connected with the use of a 

 machine, (till confining ourselves to simple exposition of these circum- 

 stance*, and avoiding mathematical explanation. Suppose a crane by 

 which a man, with a certain amount of labour, raise* five hundred- 

 weight through a height of ten feet, working all the while as if he 

 were raising half a hundredweight, by a simple pulley, from the deck 

 of a vessel to the wharf. If this five hundredweight were divided into 

 ten portions of half a hundredweight each, and if each of these portions 

 had a rope fastened to it, it would matter nothing, the manner of 

 working being equally convenient in both cases, whether he raised the 

 whole at once by the crane, or the ten portions in succession by their 

 several ropes. This supposes that there is no friction in the parts of 

 the crane, and that its wheels can be moved without any exertion when 

 unloaded. Neither of these is true, for the friction of the loaded 

 machine is considerable, and even the unloaded machine costs some 

 exertion to set it and keep it in motion. So for then the balance is in 

 favour of raising the ten subdivisions of the five hundredweight in 

 succession ; and we now see what those persons mean who say that a 

 machine is a loss of power. But if we introduce the beneficial effect 

 produced by the machine, we see that the subdivision of the weight is 

 avoided, and that the labour thereby saved may be thousands or even 

 millions of times that caused by the friction of the machine and 

 the necessity of moving its wheels, Ac. A little attention to such 

 reasoning as the preceding will prevent the reader, however unprac- 

 tised in mechanical considerations, from being led away by accounts of 

 perpetual motion [MOTION] and of machines which are to work 

 without power applied. 



The muscular power of men and animals, the force of wind, the 

 fall of water, the ex]xuisive power of steam, ic, are real powers, the 

 explanation* of which lie in the secrete of the laws of life, gravitation, 

 and chemistry. A machine is an adaptation of material elements to 

 one or more purposes, the life of which is one or other of the powers 

 juit mentioned. But nothing is more common than, in describing the 

 wonderful effects of power and adaptation united, to lay the wonder on 

 the wrong part. Thus we can imagine a person describing the progress 

 of mechanic* in the last century, by saying that " lace is made by 

 team, and mine* which would be covered with water but for the 

 application of science, are cleared for tin- miners by machinery." In 

 the first instance the (team-power is but subordinate; horses or a 

 water-mill might supply its place without any diminution of the extra- 

 ordinary part, which is the adaptation of niafliinery to thu perform- 

 ance of that which required so many and varied motions of the fingers. 

 In the second instance, common pumps, or successions of them, would 

 do a* well as the machinery > hands enough could be found 



to work them : the wonder i* the introduction of labour to any 

 amount by the [help of steam. The vulgar notion is that steam, as 

 team, can adapt itself to anything, and that machinery, as machinery, 

 can work. 



I'uWKU i >K ATT' iKNKY. [LETTER OF ATTOB!Y.] 



POWER (Law). [Uses.] 



POZZUOLANO. A material used by the ancient Roman, and even 

 by modern, hydraulic engineers for the purpose of preparing an artificial 

 cement for work* constructed in the sea. This material was o> 

 obtained from the volcanic formation* of the Bay of I'ulooli, near 

 Maple* (from which circumstance the name itself is derived) ; 1>ut 

 similar substances have been discov< r.-.| in different parts of hah ami 

 in other countrie*, and hare been applied in the arts with equal 

 miocee*. In fact the poxzuolano is a dehydrized silicate of alumina, in 

 which the silica exists in a state easily attackable by caustic alkalies, 

 and is therefore capable at once of entering into a combination with 

 the lime presented to it in the preparation of mortars, forming with it 



a double silicate 'of lime and alumina of an insolnblo chanvt. > 

 ubstances of thi* description are very commonly met with in volcanic 

 district*, wherein lava flood* of a basaltic character have been poured 

 out upon argillaceous beds. Thus in the environs of Rome itself 

 the celebrated Catacombs were excavated in a large deposit of this 

 argillaceous rand ; in the island of Sardinia ; in the sou- 

 France; in the north of Germany, near Andernach ; and no doubt 

 in countless other localities, the pozzuolano ha* been found under 

 favourable economical conditions. The German material of thi* 

 description is known in commerce under the name of (nut, or 

 terra*. 



Both the trass and the pozzuolano ore used with the most favourable 

 result* when mixed with the purest and richest hydrates of lime ; but 

 the success of their application depends entirely upon the intimate 

 nature of the mixture of the various ingredients, and upon the : 

 tion uf the hydration. These conditions can only be secured l>y the 

 most perfect manipulation ; and that again by the low price of manual 

 labour ; and it is on thi* account that the use of the natural cements, 

 or of the artificial hydraulic cements, has been latterly introduced in 

 most cases instead of that of the pozzuolano mortar* which Smeaton and 

 the earlier engineer* so strongly recommended. But it is essential to 

 remark (as was indeed before stated under MOKTAHS) that there are 

 some conditions with respect to the state of the silicates of alumina 

 entering into the composition of artificial cements, which are essential 

 for their success ; and, as far as our present means of analysis allow us 

 to form an opinion on the subject, it would seem that these conditions 

 are the most satisfactorily fulfilled when the combinations between 

 the silica, the lime, and the alumina, ore effected under great heat. 

 This is actually the case with the Portland cements ; and tin 

 different results obtained by the mixture of the volcanically dehydrized 

 silicates of alumina from those obtained by the mixture of undcrlmrnt 

 brick earths, their chemical equivalents in nearly all respects, tend to 

 confirm this opinion. It may be added that in Holland, Northern 

 Germany, and in Italy, the pozzuolano class of materials i. .-.till used ; 

 but that in England they have so entirely been laid aside of late years 

 as not easily to be obtained in the ordinary market 



Berthier gives the following analysis of the trass of Andernach and 

 of the pozzuolano of Civita Vecchia : 



PoKzuolano. 

 0-445 

 0-150 

 0-088 

 0-047 

 0-1SO 

 0-01* 

 0-040 



Total 



1-000 



1-000 



PRACTICE, a rule of arithmetic, appropriately so termed because it 

 hardly contains any new principle, but depends for its application upon 

 the memory and dexterity which the operator acquires from practice. 

 Thus, in the following simple question, " How much do 40 yards cost 

 at 18rf. a yard," some arithmeticians (unpractised) might find it neces- 

 sary to multiply 40 by 18 and divide the result by 12, for the number 

 of shillings in the answer; but a practised arithmetician would imme- 

 diately see that 18rf. is a shilling and a half, so that 40. must be 

 allowed for the shilling, and 20. for the half shilling, making altogether 

 60. More complicated examples may require greater subdivision, but 

 the method of proceeding has been completely described in the pre- 

 ceding. Suppose, for instance, it is required to find the price of 253$ 

 yards at 21. 13. 7|rf. a yard. Thu application of the rule of practice is 

 aa follows : 



At II. lit. l\i. pr yrd 253} y.rds cost 879 5 9j | 



The process hardly needs more description than is given on the left; 

 y dinVnlty is the division of the price into portions each of 

 which is a simple aliquot part of one of the preceding, and this diffi- 

 culty is to be overcome by practice. It is also to be noticed that easy 

 verification* often occur : thus, in the last process but one, it can easily 



lined that 253$ farthings is 6<. Stf. J. 

 \Vh'-n I with the factors which arc to be multiplied contain compl 



us, this rule can be easily applied by turning the money factor 

 into pounds and decimals of a pound, as in IM-HU-T. Thus, suppose 

 it is required to find the price of 22 tons 17 cwt. 1 qr. 19 Ibs.. at 

 13i. 16f. 4rf. a ton, or 13-81867/. We have then 



