INTO THE STRENGTH OF WROUGHT-IRON PLATES. 
685 
From Barlow the stren 
Box 
gths are, — ■ 
lbs. 
. . 20,000 
Beeeh . . . 
lbs. 
. 11,500 
Ash 
Oak .... 
. 10,000 
Teak. . . . . . 
. . 15,000 
Pear .... 
9800 
Fir 
. . 12,000 
Mahogany . . 
. 
8000 
Mr. Barlow, in adverting to the experiments of Musschenbrock, observes, that 
some of them differ considerably from his own, a circumstance probably not difficult 
to account for, as the different degrees of dryness have a great effect upon the 
strength of timber*. 
Dr. Robison, in speaking of the experiments of Musschenbrock, states, that we 
may presume they were carefully made and faithfully narrated, but they were made 
on such small specimens, that the unavoidable natural inequalities of growth or tex- 
ture produced irregularities in the results which have too great a proportion to the 
whole quantities observed. It is for the same reason that I give preference to 
Mr. Barlow’s results, as he observes, “ that the experiments from which they are 
drawn wei’e made with every possible care the delicacy of the operation would 
admit.” Assuming therefore that Barlow is correct, and taking the mean strength 
of iron plates, as given in the preceding Tables, at 49,656 lbs. to the square inch, or 
calling it 50,000 lbs., and the resistance of the direct cohesion of different kinds of 
timber as given by Mr. Barlow, the following ratio of strengths will be obtained : — 
Timber 
Iron. 
Ratio, taking 
lbs. 
lbs. 
timber as unity. 
Ash . 
. . . 17,000 
50,000, 
or 
as 
1 
2-94 
I’eak . 
. . . 15,000 
50,000, 
or 
as 
1 
3-33 
Fir . 
. . . 12,000 
50,000, 
or 
as 
1 
4T6 
Beech 
. . . 11,500 
50,000, 
or 
as 
1 
4*34 
Oak . . 
. . . 10,000 
50,000, 
or 
as 
1 
5-00 
Hence it appears that the direct cohesion of iron plates is five times greater than 
oak ; or in other words, tlieir powers of resistance to a force applied to tear them 
asunder is as 5 to 1, making an ii-on plate ^ inch thick equal to an oak plank of 
2^ inches thick. In the teak wood and fir specimens, which exhibit greater resisting 
powers, nearly the same rule will apply, and thinner planks, as regards the tensile 
strength, would answer the purpose. This is a circumstance which may be appli- 
cable to teak wood, but unfavourable to fir when viewed as a building material ex- 
posed to a great variety of strains, or when used for sheathing and similar purposes 
in the art of ship-building. The teak wood being timber of greater density and of 
* It has been shown by Mr. Hodgkinson, that timber, when wet, will be crushed by a force less than one- 
half of what would take to crush it when dry. It therefore follows that much depends upon the samples selected 
and the way in w'hich the timber has been seasoned. 
