NATURE 



525 



THURSDAY, APRIL 9, 18S5 



TREDCOLD'S "CARPENTRY" 

 Elementary Principles of Carpentry. By Thomas Tred- 

 gold, C.E. Sixth Edition, by E. Wyndham Tarn, M.A., 

 Architect. (London : Crosby Lockwood and Co., 1885.) 



MR. TARN has for a good many years enjoyed a 

 high reputation amongst the profession of archi- 

 tects as a writer upon the practical principles of building 

 regarded mathematically. Tredgold's treatise on Car- 

 pentry has for a very long time indeed possessed the 

 highest reputation as a much more than elementary book 

 of reference upon that important department of building 

 construction which deals with timber work : it has been 

 republished time after time in the form of the old- 

 fashioned substantial quarto which used to be in vogue 

 before we were encouraged to expect to read as we run. 

 It is quite in accordance with the fitness of things that 

 Tredgold and Mr. Tarn should come together, and the 

 English building world will scarcely require to be told 

 that the result is satisfactory. The new edition before us 

 is in fact a readaptation once more of the excellent 

 material of the old standard treatise to the changing con- 

 dition of our mechanical knowledge and skill. The 

 author's mode of treating his subject has been retained 

 intact ; and we still have the well-known sections upon 

 pressures, resistances, floors, roofs, domes, partitions, 

 centers, bridges, joints, and timber. Whether this par- 

 ticular arrangement is the best, is a question scarcely 

 worth asking, at least on behalf of the less fastidious 

 criticism of those practical designers of carpentry who 

 must here constitute the overwhelming majority of 

 readers ; but the editor has certainly not found it to be 

 any bar to the importation of new matter in his own way. 

 In one section he has introduced Prof. Clerk Maxwell's 

 now universally appreciated system of diagrams of press- 

 ures, whereby the mere application of a common drawing 

 scale to the component lines of easily constructed geo- 

 metrical figures saves all further trouble and uncertainty 

 in ascertaining the precise strains which the several 

 members of a truss have to bear. In other sections the 

 accepted formula; of calculation, given only empirically 

 by Tredgold, are mathematically demonstrated. The 

 familiar tables of strength which supply the values of 

 constants are " corrected " to accord with recent experi- 

 ments more delicately and adroitly conducted, and several 

 new tables of the kind are added. The consequent 

 revision of Tredgold's "rules" and tables of scantlings 

 has been thoroughly and carefully done ; and various 

 modes of more advanced construction are duly developed. 

 That difficult subject, the theoretical thrust of domes — for 

 in practice there ought to be none — is taken in hand 

 mathematically, and a short chapter is added on stone 

 vaulting. The important items of scaffolding, shoring, 

 coffer dams, and so on, have been also introduced. The 

 remarkable timber bridges of America — rough and ready 

 science of the best — have been taken account of as they 

 ought ; and certain amendments which are made in 

 respect of the plates serve in a reasonable measure to 

 substitute new trussing for old. Lastly, the description 

 of the nature and properties of timber is largely modified 

 Vol. xxxi.— No. 806 



to meet the advanced knowledge of the day. With all 

 this, the Tredgoldian character of the treatise is dutifully 

 preserved ; and so we may say it ought to be, for to 

 modernise Tredgold too much would certainly not im- 

 prove him. One of the prominent merits of the work 

 consists in the unusually large number of illustrative 

 plates, all to a useful scale. If these do not represent 

 many of the more modern designs in timber work, they 

 frequently offer examples to the student which are all the 

 better in one respect — they exemplify that substan- 

 tiality of construction which it is too much the tendency 

 of scientific precision almost to discourage. It is a good 

 maxim in carpentry as in most other departments of 

 building, to make the structure not only strong but 

 stronger than strong ; and Tredgold always leans in that 

 direction. The word economy is much employed amongst 

 us ; but, whereas its original and proper signification 

 pointed only to skilful administration, its meaning with 

 us is very much like mere parsimony. Waste of material 

 is the bugbear of our builders, and almost still more of 

 our architects. It need not be denied that mathematical 

 science is in a certain way provocative of such parsi- 

 mony ; indeed, lightness of construction is regarded as 

 an academical virtue in both architecture and engineer- 

 ing. But a moment's reflection ought to satisfy alike the 

 most scientific and the least that true science is as much 

 averse to parsimony of substance as common sense is. 

 The strength of building materials can only be deter- 

 mined by extremely delicate experiments upon " break- 

 ing" strains, from the results of which the " safe" strains 

 have to be deduced by estimate ; and this, no doubt, 

 becomes matter of opinion. The question is, what pro- 

 portion of the breaking strain shall be recognised — almost 

 arbitrarily — as the safe strain ? With the single exception 

 of iron, timber is the material with reference to which 

 this matter of opinion is the most definitively settled. 

 The reason is this : — The breaking strain must be instan- 

 taneously applied ; this is essential to precision of tabula- 

 tion. The safe strain is that proportion of this instan- 

 taneous breaking strain which the material will bear 

 permanently without any risk of its elasticity being event- 

 ually overcome and a commencement made of that dis- 

 turbance of the structure of the material which, once 

 begun, increases in a geometrical ratio until the end is 

 ruin. It is accepted, therefore, that the proportion of an 

 ascertained instantaneous breaking strain which has to 

 be recognised as the limit of a permanent safe strain is 

 one-third, one-fourth, and so on, according to the cha- 

 racter of the material. What does this mean ? It means 

 that a greater strain than this proportion would in time, 

 with one accidental circumstance and another, produce a 

 commencement of instability. Perhaps it is to be re- 

 gretted that this question is still disposed of so empiric- 

 ally as it is ; we might at least in these days have 

 express observations made and reduced to what system 

 might appear. Tredgold's rules turn very much upon 

 the manifestations of flexure ; and this, of course, is not 

 only another way of dealing with the matter, but one 

 which affords at any rate a basis upon which mathe- 

 matical formula: may be arrived at. On the whole, 

 Tredgold is an old-fashioned writer, empirical and prac- 

 tical ; but he is none the worse for that, perhaps all the 

 better. Mr. Tarn has accepted the duties and responsi- 



