496 



CARPENTRY- 



Theory of are those of Buffon, recorded in t^eAtm. Paris dead. 

 Carpentry. 1741 an( j 1742. Duhamel has given a great many 

 S ^"Y* / in his Trails stir Ic Conservation dc Bois ; but they 

 have omitted that primary and most essential object 

 of their inquiry, the direct and absolute strength of 

 the timber employed, by which it resists extension 

 and compression. Experiments made on the trans- 

 verse strength are so much modified by the different 

 degrees with which timber resists those strains, that 

 little satisfactory reasoning can be founded on them, 

 however numerous they may be, or however careful- 

 ly performed and faithfully described. 



Muschenbroek is almost the only author who has 

 enabled us to judge of the accuracy of his experi- 

 ments ; he details those on the ash and the walnut, 

 stating the weights requisite to tear asunder slips ta- 

 ken from the four sides of the tree, and on each 

 side in a regular progression from the centre to the 

 circumference ; and he says all the others were made 

 with equal care. We cannot, therefore, see any rea- 

 son for distrusting the results. Yet they are consi- 

 derably higher than most other writers have given. 

 The probability is, that they were made in such 

 small specimens, that any want of soundness, which 

 would of course be easily detected, induced him to 

 reject the example, although absolutely requisite to 

 give a fair average. Perhaps, too, he gives us the 

 utmost load which may be borne rather that which 

 will break the bar in a few seconds. This is much 

 more than can be safely trusted to the beam. 



In the English language, we have hardly any thing 

 to guide us. The Royal Society appear at an early 

 period, to have paid some attention to the subject ; 

 but they have recorded nothing.- Emerson gives a 

 list said to be derived from experiment ; but we are 

 not even told, whether it refers to the transverse or 

 the longitudinal strength of the material. To these 

 we shall add some related by the late Dr Ander- 

 son of Glasgow, a man fond of experiments, and pro- 

 bably faithful in relating them. 



Its amount. 



Absolute strength of an inch square drawn in length, 



Poplar .... 5.5OO 



Cedar 4,880 



Box 7,850 



Yew 7,850 



Holly 5,000 



Crab 5,000 



Cherry 4,760 



Hazle . . 4,760 



Asp 4,290 



Birch . 4,290 



According to Emerson, a cylindric rod of good clean 

 fir, an inch in circumference, drawn in length, bears 

 400 Ibs., and a spear of fir, two inches diameter, 

 bears 7 tons, but no more ; a rod of good iron, one 

 inch in circumference, bears nearly 3 tons weight, and 

 a hempen rope of one inch in circumference carries 

 1000 Ibs. 



He also gives the following rule : Square the dia- 

 meter expressed in inches, and multiply for 

 Fir, by 8f 

 Rope, by 22 

 Iron, by 106 



The result gives the load in cwts., which may be 

 safely trusted. , 



It is to be observed, that all these results are de- 

 rived by proportion from the strength of very small 

 specimens ; and this may account for such great ano- 

 malies. It is highly probable, that the cohesion of 

 timber, especially that of which the fibres are much 

 intertwisted, such as oak, increases in the thicker 

 pieces in a ratio which is greater than that of the area 

 of the section. Such fibrous bodies, by being drawn 

 in length, are strongly compressed together, and those 

 weaker fibres are more firmly retained, which, by break- 

 ing at their finest part, might otherwise tear out. 



This is indeed always the way in which such bo- 

 dies fail, viz. by the fibres sliding out from among 

 each other, and not by an absolute snap. In this re- 

 spect, therefore, timber may be compared to cordage ; 

 and there we have several examples of the truth of 

 this principle. 



Duhamel mentions, that a 6 thread rope bore 

 631 Ibs., but that thicker cords bore more in propor- 

 tion. We have arranged the results below. 



Though the increase in these experiments be not 

 regular, yet it may be sufficient to convince u? ? that 



the larger cords, and consequently the larger timber, 

 otherwise equally gQwnd, will be in proportion strong* 



