ON ARCHITECTURE AND CARPENTRY. 131 



climates, a steep roof is required on account of falls of snow, which 

 greatly increase the lateral thrust of the rafters ; for the horizontal force 

 exerted by a roof is always proportional to the length of a line perpendi- 

 cular to the rafter, descending from its extremity till it meets another 

 similar line drawn from the opposite rafter ; and this perpendicular is 

 obviously much increased when the roof becomes very flat. But for bear- 

 ing the transverse strain which tends to break the rafters themselves, a low 

 roof is stronger than a high one, supposing the number of braces and 

 queen posts equal on both ; for if we have to support a given weight by a 

 beam or rafter, whether it be placed in the middle, or equally divided 

 throughout the length, we neither gain nor lose force by lengthening 

 the beam and raising it higher, while the horizontal span continues the 

 same, since the obliquity lessens the effect of the weight precisely in the 

 same ratio that the length of the beam diminishes its strength ; but by 

 lengthening the beam we also add to the weight which is to be supported, 

 and we thus diminish the strength of the roof. It must be observed, in 

 calculating the strength of a rafter, that the slight flexure produced by 

 the transverse strain, has a material effect in diminishing its strength in 

 resisting a longitudinal force ; and this diminution must be determined 

 according to the principles that have been laid down respecting the equili- 

 brium of elastic substances. 



Wooden bridges, and the temporary centres on which arches of stone 

 are supported during their construction, depend nearly on the same prin- 

 ciples as roofs : the external parts usually support a thrust, and the 

 internal act as ties ; but the abutments are generally capable of withstand- 

 ing a horizontal thrust without inconvenience, so that by their assistance 

 the strain on the ties is considerably diminished. Great strength may 

 also be obtained, where it is practicable to support each part of the centre 

 by two beams, in the direction of chords, bearing immediately on the abut- 

 ments. (Plate XIV. Fig. 178, 179.) 



The various articles of household furniture belong to subordinate 

 branches of carpentry, but their form is in general more accommodated to 

 convenience and elegance than to strength and durability. Yet, even in 

 making a chair, there is room for error and for improvement ; the same 

 principles that direct us in framing a roof are capable of application here ; 

 but if they were implicitly followed, they would lead us to the employ- 

 ment of bars crossing each other in an inelegant manner. Doors, gates, 

 locks, and hinges, are either parts of the carpenter's employment, or 

 appendages to his works ; and it is possible that, by attentive considera- 

 tion, improvements might be made in all of them. Mr. Parker has de- 

 voted much time and labour to the subject of gates, with their hinges and 

 fastenings, and has presented to the Royal Institution a very useful col- 

 lection of models, which show the result of his investigations.* 



* Parker on Gates, Lond. 1801, Rep. of Arts, ii. II. 50. 



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