178 REPORT—1857. 
It is scarcely necessary further to insist upon the importance of the Mechanical 
Section, It is obviously the interest of public men, no less than of private individuals, 
to pay increased attention to Mechanical Science. If this Section can contribute ever 
so little to bring out new facts, or to direct attention to facts already known, it will 
have rendered good service. 
A detailed Model of the Boyne Viaduct which carries the Belfast Junction 
Railway over the River Boyne at Drogheda, with a description of it, and 
the Principles of its Construction. By James Barton, C.E. 
The dimensions of this work are,—height above high water, 90 feet to soffit of bridge; 
open of centre span, 264 feet ; of side spans, 140 ; besides fifteen stone arches of 61 feet 
span each. ‘The three centre spans are crossed by wrought-iron lattice beams designed 
by the author; and the chief feature of interest connected with this work is the mode 
in which the results of careful investigation of the strains on every bar and plate have 
been practically applied. Having ascertained the maxima strains, whether tensile or 
compressive, that each bar could be subjected to by the weight of the structure, com- 
bined with the passing load entire or in part, the areas of iron and the form of the 
parts were then designed and made proportionate to their maxima strains; the limit 
assumed being that no part should ever be subjected to more than 4 tons per inch of 
iron of compressive, and 5 tons per inch of tensile strain, and that the parts under 
compressive strain should be so arranged and braced that they should not yield by 
flexure. 
The three spans are crossed by a continuous beam ; and the author explained how 
this arrangement, whilst it was economical and decreased deflection, involved much 
more complicated calculation. He showed when the points near the top and bottom 
of the beams, called the points of inflection, were under different states of the load, and 
how these points travelled along as the load passed over, these points being points at 
which there is no strain of either tension or compression, the top and bottom on 
either side of these points being under opposite kinds of strain. 
The iron used for this structure was much less than ever the same spans had been 
crossed with before by any Girder Bridge, and was at the same time as strong as any. 
The total iron used was 740 tons, and the cost complete was about £24 10s. per ton 
as it stands, exclusive of scaffolding below the ironwork. The iron was from Staf- 
fordshire ; that subject to a tensile strain, of the quality known as “ best best;” that 
only subject to compression, of the quality termed “best iron.” The correctness of 
the calculations, had been proved in some remarkable ways: the points where theo- 
retically there should be no strain in the top under a uniform load having been deter- 
mined for the large span of 264 feet, the author, after the bridge was relieved of its 
supports, severed the top of the main beam in these places, and the strict mainte- 
nance of equilibrium in the structure proved that no strain was then passing through 
the open joints of the ironwork. Again, it was calculated that when a load of 2 tons 
per foot forward would be put upon the centre span, in addition to its own weight, 
the points of inflection of the side spans would pass the ends of the beams, the result 
of which would be that the side spans ought not to rest any longer on the abutments 
of masonry, but stand out as overhanging beams from the piers between them and the 
centre span. ‘This actually took place; and when the centre span was thus loaded, 
the ends of the side spans were found to have lifted 1 inch off their bed plates and 
rollers ; nor did they descend until a locomotive from the centre span travelling along 
the side span from the centre came within about 40 feet of the end. The bridge had 
been tested with 1100 tons and in a variety of ways, and the results were most satis- 
factory. 
On Coal-burning Engines. By J. 8. BEATTIE. 
On Electro-Magnetic Engines. By J. 8. BEATTIE. 
