114 Account of the Roof of the Kdsipur Fovndery. [Feb. 



roof and looking in at the upper window of either end, the same effect 

 was still visible, though in a diminished degree, and we were not 

 convinced that it was a deception, until Major Hutchinson at our re- 

 quest caused an actual measurement to be made by a perpendicular 

 wooden batten from an accurately adjusted level on the stone floor. It 

 was then proved that there did not exist a difference of level even to 

 the amount of a tenth of an inch. Whence arises the illusion ? Is it 

 that the eye, judging of directions by comparison with other objects, 

 and having the numerous lines of the pent roof inclined in opposite 

 directions to each half of the horizontal rods, is thus perplexed in its 

 estimate ? the ruler experiment is opposed to such an explanation. It 

 may, perhaps, be owing to the effect of light from the upper windows, 

 which frequently gives a curved appearance to wooden beams from the 

 decrease of illumination from side to centre. If the phenomenon re- 

 semble the effect of the eyes in a portrait always looking the same 

 whencesoever viewed, or the curves formed by spokes of a wheel pass- 

 ing a railing, as has been suggested, the effect should admit of a rigid 

 explanation, and we may hope to obtain it from some one of our readers 

 who may have time to investigate this singular deception. 



Note. — The mode of calculating the strain upon the iron rods in the ahove 

 account is familiar to engineers, but it may be acceptable to others (for in In- 

 dia every man is his own architect), to be furnished with a correct table of the 

 strength of timbers and iron : the following extract therefore from Mr. Barlow's 

 report on the subject to the British Association in 1833, may be acceptable. He 

 prefaces it by a precis of the various opinions and theories hitherto formed to ex- 

 plain the strain and process of fracture, and strongly recommends Tredgold's 

 Treatises on Iron and on Carpentry. There is now no longer any disagree- 

 ment on the leading principles connected with the strength of materials, excepting 

 such as arise from the imperfect nature of the materials themselves, which fur- 

 nish different results even in the hand of the same experimenter. 

 Formula relating to the ultimate Strength of Materials in cases of Transverse 



Strain. 



Let /, I, d, denote the length, breadth and depth in inches in any beam, w 



I w 

 the experimental breaking weight in pounds, then will ~~JJ~ = s De a con- 

 stant quantity for the same material, and for the same manner of applying the 

 straining force ; but this constant is different in different modes of application. 

 Or, making S constant in all cases for the same material, the above expression 

 must be prefixed by a co-efficient, according to the mode of fixing and straining. 



1. When the beam is fixed at one end, and loaded at the other, 



b d* 



2. When fixed the same, but uniformly loaded, 



1 I w 



T Yd 2 = s ' 



