374 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[Dkc. 



" From Mr. Donkin's investigation of the subject, he was led to con- 

 dude that it is quite impossible to protluce a screw which shall be abso- 

 lutely free from error, when microraetricully proved ; and in lS2;i, he was 

 in consequence led to consider that as Mr. Maudslay's method of the bent 

 lever and inclined straiglit bar, would compensaie the error of total length 

 in a nearly perfect screw, a similar mode mi^ht be applied to all tlie inter- 

 mediate errors, by the employment of a ciirre, experimentally obtained by 

 the method of continual bisection employed in hand dividing. 



" It having been explainer! in reference to the diagram,* that the incli- 

 Dation given to the bar would reduce the ell'ective li'iigth of a screw, and 

 the reverse inclination would increase it, Mr. IJonkin considered that from 

 the observed fact of one half of the screw (as estimated by counting the 

 number of threads) being generally too coarse, and the other half loo line, 

 the compensation would require the one half of the bar to be inclined to 

 the right, as in the diagram, and the other half to the left; in fact, thus 

 bendmg ihe right line into an obtuse angle. 



" Extending this mode, upon the presumption that the quarters, eights, or 

 •ixteenths of the screw were also dissimilar, the bar would require many 

 flexures instead of tlie one only, giving to it a more or less zig-zag character, 

 or rather that of a gently undulating line. The undulations being propor- 

 tioned experimentalli/, to effect such compensations, as should add to the 

 movement of the upper platform or supplementary table, where the screw 

 was too line, and subiract from its motion, where the screw was too coarse ; 

 so as, from a screw known to be slightly irregular, to produce the divisions 

 of a scale, or the thread of another screw, considerably nearer to equality. 



" lie carried out this project in 182G, and be has satisfactorily proved the 

 existence of a correctional method, which is within reach of any clever 

 workman who will devote sufficient patience to the adjustment of the engine, 



and which latter will be now briefly explained To effect the com- 



pensation, the platform or table consists of an upper and lower plate, which 

 are capable of a small independent motion. The lower plate carries the ful- 

 crum of the bent lever, whose arms are at right angles and as fifty to one, 

 the lever moves in the vertical plane, so that its longer arm lies by gravity 

 alone on the curvilinear edge of the compensation bar ; the upper platform 

 is pressed endlong against the shorter arm of the bent lever, by a spring 

 which always keeps them in close contact. 



" The compensating bar, which is of the length of the screw, or 24 inches, 

 has 48 narrow slips of metal placed like the keys of a piano-forte, each 

 having an appropriate adjusting and fixing screw, by which the ends of the 

 pieces may be placed in a continuous line, or any of them may be placed 

 above or below the line, as required in the following mode of compensation. 

 For chanee of total length and adjustment for temperalure, the curved bar 

 is more or less inclined, as in the former example, except that it is placed 

 edgeways or vertically ; it is attached to the outside of one of the rails, by 

 a pivot which intersects the one end of its curvilinear edge, and the other 

 end is raised or depressed by a screw, which effects the adjustment for tem- 

 perature. 



"Conceiving the length of the guide-screw divided into 48 equal parts, de- 

 noted by the figures to 48, it would be first ascertained by two fixed mi- 

 croscopes, if the halves 'of the screw, measured from to 24, and from 24 

 to 48, were absolutely equal quantities ; if not, the central slip or finger 

 would be raised or lowered until on repeated trials tlie due correctional 

 movement was applied to the table. The two halves would be similarly bi- 

 sected and corrected in the points 12 and 36, and the quarters again bisected 

 in 6, 18, 30, and 42; and the eighths when also bisected, would extend the 

 examination to the points 0, 3, 6, 9, &c., to 48. The easiest method is to 

 compare the path of the slide, with the divisions of a superior scale, fixed 

 upon the shde or platform of the machine. 



" It would now be needful to divide the whole into three parts, by the 

 comparison of the spaces from to 10, from IG to 32, and from 32 to 48, 

 the points 10 and 32 being adjusted until exactly equal, which is the most 

 difficult part of the work ; and then these three distances being bisected 

 four times, every point of the 48 would have been examined, and some of 

 them twice over. These adjustments having been repeatedly verified, during 

 which a very frequent recurrence to the total length is imperative the con- 

 cluding step is to file off the corners of the 48 slips very carefully, so as to 

 convert them into a line with undulations, slight it is true, but which repre- 

 sent fifty-fold the actual errors in the guide-screw ; and therefore shift the 

 table simultaneously with its general traverse, so as to apply the exact cor- 

 rections for inequality, at every point examined and found to be in error. 



" liut the term error must be received in a very restricted sense, as it de- 

 serves to be noticed that Mr. Donkin first used a screw made by Mr. Mauds- 

 lay, and the maximum deflection of the curved edge of the compensation 

 bar from a straight line was very nearly the ei()htk of an inch, indicating the 

 maximum error of the screw to have about the 4n0th part of an inch ; and 

 as the curve was nearly limited to a single undulation, or a bill at one 

 end, it may be presumed this minute error was in part attributable to a 

 difference in the material, a source of perplexity from which no care is a 

 sufficient protection. The dividing engine was employed as a traversing 

 lathe in cutting a new screw, and which, although it had the advantage of 

 the compensation, onlv reduced the error of the new screw to about one- 

 third the quantity of that of the first ; as shown by the new curve assumed 

 by the compensation bar, its deflection being t}j; of an inch, when re-adjusted 

 in the tedious and anxious method described. 

 ," lu the past year, 1842, Mr. Donkiu has ma de a similar bnt en - 



* GiTeu in tLe lust extract. 



larged dividing engine. The length of traverse of the new machine 

 is 42 inches, the screw has 40 threads to the inch, the compensation 

 bar is as UO to 1, and the value of one single tooth in ihe counting 

 wheel is equivalent to the C0,000lh part of an inch ; that of the first ma- 

 chine having been the 30,000ih part. It is to be hoped that Mr. Donkin 

 will complete his labours, by publishing a detailed account of these ma- 

 chines, the latter of which, in particular, exhibits throughout its structure 

 a most refined contrivance and execution, of which no adequate idea can 

 possibly be conveyed within the limits of this slender notice, nor without 

 exact drawings of the details, to the arrangement of which great attention 

 has been bestowed." 



The work is in plan one of the most extensive of all those which form 

 the literature of praclical science, and forms an invaluable addition to it. 

 We may probably again refer to it. There are two papers by Professor 

 Willis ; we should like to know whether they have been originally com- 

 municated to this work, or are merely reprinted. They are inserted with- 

 out any iutroductiou. 



The Assistant Engineer's Railway Guide in Boring, Ifc, preceded by 

 Practical Memoranda on Specifications. Illustrated by 60 woodcuts and 

 plates. Hy \f. Uavis Haskoll, Civil Engineer. Williams, Strand, 1846. 

 8vo. pp. 130. 



So many books are now published on various branches of the practice of 

 railway engineers, that we naturally look to the commencement of each 

 new publication fo learn the author's specific object, and how his produc- 

 tion differs from the rest of its class. The present work has not, however, 

 any preface or introduction : it indicates as far however as may be learned 

 from the title of the book, and an examination of its contents, Mr. Haskoll'a 

 object has been to set before the assistant engineer a concise, but at the 

 same time, explicit directions for his guidance, in examining the progress 

 of a railway, and in ascertaining that the contractors' work is satisfactorily 

 performed. The first, and one of the most useful chapters, gives an ab- 

 stract of the clauses which ought to be contained in a specification for the 

 construction of a railway, and the general conditions of contracts. The next 

 chapter treats of railway borings, and about one-fifth of the whole work is 

 occupied by a full account of the tools and operations requisite in boring 

 to ascertain the nature of the strata which a railway traverses ; this chap- 

 ter concludes with an account of the economic properties of various kinds 

 of English building stones. The third chapter treats of the methods of 

 setting out the centre line and curves of a railway. This is followed by 

 minute directions for setting out slopes, making land plans, &c. The fifth 

 chapter explains the method of setting out foundations, and affords eome 

 hints which will be very valuable to those eugaged in over-looking tlie 

 construction of brick arches and masonry. Mr. Haskoll gives a very bad 

 accouut of bricklayers. They seem a slippery set of fellows, and require 

 as much watching as horse-dealers. 



The general duections for securing firm foundations are good in 

 themselves, but not perhaps sutSciently extended : instead of referring 

 to other works, such as Hughes's papers on Foundations, Godwin on 

 Concrete, Vicat on Cements, and papers in the " Civil Engineer and 

 Architect's Journal," the author might have furnished a useful colla- 

 tion of information from those sources. At the end of the book are a table 

 of gradieuts, and that most valuable appendage— a good index. The ar- 

 rangement of the book is very clever ; the paragraphs are numbered and 

 reference to them is greatly facilitated by the frequent marginal references 

 which occur on every page. The author has saved the student much 

 trouble by bringing together a great deal of information, scattered about 

 different publications, and his task is performed in a successful manner. 



The following is, with some omissions, the abstract of the clauses which, 

 ought to be contained in a specification drawn up on the part of railway 

 company, for the direction of contractors. For precedents of the general 

 conditions of contracts, the author refers to those of Stephenson, Brunei 

 and other engineers, in Brees' Railway Practice. The perusal of the ex- 

 tract below will give to the student a good general idea of the practice of 

 railway construction, and of the points to be attended to in overlooking the 

 progress of the works. 



" Strong general clauses in a specification are unjust and arbitrary to- 

 wards a contractor, where they are only iuteuiied to shelter au engineer 

 from neglectful omissions, and from due responsibility, aud where full and 

 particular explanations and instructions are not given, for the construction 

 of the various works, as they occur along the different contracts. But 

 where they refer to the manner of the work, general workmanship, to the 

 kinds and qualities of materials, unless otherwise specified in particular 

 place, tliey canuot be objectionable. General couditions to contracts should 

 be added lo protect companies from trouble ami annoyance, resulting from 

 the depredations and reckless conduct of workmen, times of payment, re- 



