'proposed io be erected at Montrose. X5t 
Good English iron will bear for any length of time, without 
stretching or altering its texture in any respect, at least 8 tons 
When a rope or chain hangs perpendicular, and carries a weight at its ex- 
tremity, it is evidently stretched by a force exactly equal to the weight it- 
self, because the cohesive strength of the rope being always exerted length- 
ways, this when the rope hangs perpendicular, is directly opposed to the 
gravitating action of the w 'eight, and these opposite forces producing an equi- 
librium, must necessarily be equal. But when the rope is made fast at its 
extremities to two opposite points of suspension, and the weight suspended 
between them, the case is quite different : Here the rope cannot hang verti- 
cally as before, but branches off from the point where the weight is attached 
tov/ards each point of suspension. It thus divides itself into two distinct por- 
tions, each of which now bears-its share of the load : — and the weight being thus 
sustained by two ropes instead of one, this, it may be imagined, should reduce 
the strain upon each one-half, and so it undoubtedly would, if each rope Avere 
to hang perpendicular. If the points of suspension were brought together, 
and the rope thus merely doubled, then it is evident that each half as it would 
only bear, so it would only be strained with the half of the weight. But 
when the points of suspension are removed from each other, and the rope 
spans the intermediate distance, then, as I have already remarked, the case 
is quite different ; since the rope, instead of hanging vertically, must incline 
from the points of suspension towards the point where the Aveight is attached. 
'But as the weight continues to draAv directly downwards, and the rope to 
exert its strength directly lengthways, the forces are hence no more directly 
opposed to each other. Each branch of the rope bears obliquely upon its 
object, and before these indirect actions can become a match for the Aveight, 
which continues invariably to draAV in the line of the perpendicular, they 
must necessarily acquire additional force, in proportion to their obliquity. If 
the rope, then, has just strength enough to bear the weight , when the points of 
suspension are brought together, it v/ill be sure to give way the moment thev 
are separated; and if it has sufficient strength to cany the Aveight, notwith- 
standing of this separation, each branch Anil yet be strained by a force, de- 
pending not merely on the weight itself, but also on the obliquity of the rope ; 
the more oblique this is to the direction of gravity, the more will it be strain- 
ed, according to the well known laAA^s of oblique forces. 
Such, then, is the reason of that remarkable strain which Are obsen^e in 
flat arches, Avhether suspended or raised, and of that moderate tension or com- 
pression which Are obtain by increasing the depth or height of the curve. In 
the former, the arch hangs or stands every Avhere extremely oblique to the 
direction of gravity ; and the strain, therefore, on every part cf it, must 
greatly exceed the natural w eight which it supports. In deeper or higher 
curves, again, the arch hangs or stands dess oblique to the perpendicular, the 
forces of cohesion or of compression, and the force of gravity, are hence more 
directly opposed to each other, and are therefore more nearly equal. In these 
eases, the A\ r eight is no doubt disposed 0A r er the AAdiole length of the arch, in- 
stead of being accumulated in the centre; but the effect of this is merely to 
