342 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL 



[October, 



Sittiiigljoiirnp, are ofa very good qu-.ility and a fine yellow colour ; 

 stone-coloured ones are made near Ipsvvicb, and have been largely 

 enjployed in the outside walls of some of the new churches of the me- 

 tropolis. At Cheslmut, in Hertfordshire, is a bed of malm earth of 

 the finest quality, no less than tvveiUy-tive feet in depth; from this are 

 made the best small kiln-burnt bricks, called paviers. They are hard, 

 absorb very little water, and are used for paving the floors of stables, 

 wash-houses, Sec. They have entirely superseded the use of Dutch 

 clinkers, which formerly were imported from Holland in large quanti- 

 ties. The red sandy bricks called Windsors are made at Hedgerley. 

 There is a considerable exportation of bricks from London ; many 

 being sent to the West Indies, to Quebec, and to other colonies.* 



Tiles, from the purpose to which they are applied, namely, roofing 

 houses in order to shoot ofVthe rain, require a texture as compact as 

 can be given to them, consistent with a due regard to economy. The 

 fattest and most unctuous clays are, therefore, those which answer the 

 best, especially if free from gravel and the coarsest sand. The price 

 of tiles, comiiared with that of bricks, is such that the manufacturer 

 can afford to dry them under cover; while, being not more than one 

 quarter of the thickness of bricks, the drying is more speedily per- 

 formed, and with far less hazard of warping or cracking: the same 

 also is the case with the baking. Sand is added to the clay, but spar- 

 ingly ; for if, on the one hand. It prevents the ware from warping, yet, 

 on tiie other hand, it increases the porosity, which is a fault especially 

 to be avoided. The general manipulations of grinding the clay and 

 tempering it are analogous to those already described for making 

 bricks; but more pains are bestowed in getting it to the utmost de- 

 gree of plasticity so as to allow of its being rolled, like dough, iuto 

 cakes of a proper thickness, which are afterwards brought to the re- 

 quired shape by pressing them into a mould. 



ON THE PERCUSSIVE ACTION OF STEAM. 



Our correspondent C.S. ui the last number of the Journal has taken 

 an observation which we made in the first part of our article on this 

 subject in the August number in a wrong light; nor should we have 

 expected him to have attached so much importance to that observa- 

 tion after reading the rest of the article, from which he woald have 

 seen that, if we thought it unfair of Mr. Farkes to attribute all the ad- 

 vantage of percussive action to Cornish engines, and none to others, 

 we also considered the amount of that advantage to be equal to nothing, 

 which wc think clearly demonstrated by our reasoning ; so that we 

 cannot exactly be of opinion that Mr. Parkes furours the Cornish en- 



fines, simply by considering that the percussive force of steam is only 

 eveloped in them. The remark that this force should be developed 

 in a greater degree iu Locomotive engines does not necessarily imply 

 that it should be developed favourably ; for, by reason of the lead 

 given to the slide valve in those engines, the steam is first let on to 

 the back of the piston, and its percussive force would therefore tend, 

 as our correspondent justly observes, "to impede the engine, if not 

 stop it altogether." 



The rest of our correspondent's remarks, since they have for object 

 "to show that the Cornish single-acting engines are the only ones at 

 present in which the percussive force of steam could act with any very 

 great advantage, and that the locomotives are the very worst in which 

 it coidd be used as a motive force," and thus suppose the fact of its 

 advantageous action in the former to be already established beyond all 

 question, cannot be regarded as an answer to our article above men- 

 tioned, but merely to the single remark already alluded to, and to 

 which he has, as we have shown, attributed a meaning we never in- 

 tended it to convey. It is difScult to assign a reason for his replying 

 to the least important portion of our article, and passing over the main 

 argument in silence. — Is it that he considers the question of the pro- 

 priety of applying the principle of Percussion to the action of the 

 steam as above discussion ? — This was not very reasonable, since he 

 has, so far as we are informed, the authority of but one writer, the in- 

 fallibility of whose theories has not hitherto been established by ex- 

 perience ; in proof of which, see the Count de Pambour's paper On 

 Momentum proposed by Mr. Jos i ah Parkts as aj\kosurtof the Mt- 

 chaiikal EJJ'tct of Locomolire engtnes, and our Reviews of Mr. Parkes' 

 paper on the same subject, in the Journal of last year, page 102. We 

 must, however, assume this to have been our correspondent's motive 

 for abstaining from any discussion of the principle of the percussive 

 action of steam, as otherwise we should be reduced to the alternative 



■^ The ttholc number of bricks made in Great Britain and Ireland in the 

 year 1835, on which the excise duty was paid, was 1380 millions. 



of either supposing that he did not understand the reasoning by which 

 ne demonstrated, or attempted to demonstrate its fallacy, or uat he 

 followed Paddy's plan of beginning at the end. Be this as it may, the 

 following remarks may perhaps induce C. S. to modify his opinions in 

 some measure. 



He observes that, " in a common double-acting rotative engine, 

 where the steam is let into the. cylinder when the crank is just passing 

 the centre, it is evident that any percussive force of the steam striking 

 upon the piston could not by any means have any eilect in turning the 

 crank." But he states farther on that "the action of this force is 

 avoided in this case, as well as in that of the Locomotive engine, by 

 the gradual motion of the slide, for as soon as the slide begins to open 

 the steam way, the steam rushes into the cylinder, and strikes upon 

 the piston, but with very little effect, on account of its being so much 

 wire-drawn in consequence of the sm.dl size of the opening at first." 

 It would naturally be inferred from this latter observation that our cor- 

 respondent supposed the percussive action of the steam to be confined 

 to the moment when the valve begins to open, in other words, that it 

 is only the Jirsi portion of the steam which has any percussive action, 

 and that this action is communicated instantaneously to the piston the 

 moment that portion of steam passes through the valve ; which, if it 

 were tiue, would obviate the "development of percussive action in 

 Coruish single-acting engines as well as iu those above mentioned ; 

 for the steam valve of a Cornish engine, though opened more suddenly 

 than the slide valve, is nevertheless not opened instantaneously, but 

 mere or less gradually. If, on the other hand, we assume the develop- 

 ment of this action to occujjy some time, however brief, so as to allow 

 of the ("peiiing of the valve of the Cornish engine, (wduch is equally 

 necessary for the double-acting rotative engine), then must we also 

 admit, not only that there is percussive action in the latter as well as 

 iu the former, but also that this action must assist in turning the crank, 

 which will have passed the centre before it has ceased to operate. 



We do not agree with C. S. in the opinion that " in order to render 

 the percussive force of steam available to its fullest extent as a moving 

 power iu single-rcting pumping engines, it would be necessary to 

 have some medium interposed between the direct action of the steam 

 on the piston and the pumps; so as to convert the ever-varyiug pres- 

 sure on the piston into a regular and steady pressure on the plunger of 

 the pump;" for the condition of a constant pressure on the pump 

 bucket, is by no means indispensable, as the effect of a diminution of 

 pressure on the steam piston, supposing such medium not to exist, 

 would be simply a corresponding diminution of the velocity, or of the 

 acceleration of the bucket and column of water, which would by no 

 means affect the application of the percussive force of the st'am. This 

 column of water is, liowever, considered by C. S. as a medium inter- 

 posed between the direct action of the steam on the piston and the 

 pumps, which is curious enough, since the action of the steam works 

 the pumps, and these raise the column of w'ater. 



We cannot make out that our correspondent's remarks have in any 

 degree shown, as he supposes, "that in the Cornish engine we might 

 use the percussive force of steam as a moving force to a very con- 

 siderable extent;" for such negative evidence as that which he re- 

 produces from Mr. Parkes' paper, viz. that the duty performed by 

 those engines is greater than he could otherwise account for, cannot 

 be admitted as rigorous. What is required of the supporters of Mr. 

 Parkes' opinions is a direct proof that in Cornish or any other engines, 

 the steam develops a power, by means of percussion, in addition to 

 that measured by its elastic force, without which it is idle to enter 

 into any discussion respecting the comparative fitness of different 

 kinds oi" engine for the development of this additional power. 



ON THE POWER OF THE SCREW. 



We have received a letter from Mr. J. R. Cussen, in defence of the 

 views set forth by him in a communication published in our Journal 

 for May last, and which were partially refuted by a correspondent in 

 the July number. We cannot insert this letter, since it is evident that 

 the writer is in error on all points ; but for his information, and the 

 information of any other persous who may be led into error by his 

 arguments, we sliall point out clearly in what his mistakes consist, 

 and how they have probably arisen. 



As to his'first objection to the theory laid down by the Rev. Mr. 

 Bridges, our July correspondent alluded to above is undoubtedly 

 right with regard" to the meaning of rf, which' signifies the heigfU of 

 the inclined plane, or distance between the top of one coil of the 

 thread of the screw and the top of the next, which is the distance 

 through which the resistance is moved in one revolution of the screw. 



