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POPULAR SCIENCE REYIEW. 
MECHANICAL SCIENCE. 
The successful opening of the Suez Canal and of the Pacific Railway, 
involving an immense change in the direction of the great currents of oceanic 
traffic, mark an era in the achievements of mechanical science. For Lon- 
doners, the completion of another section of the Thames Embankment, of 
the noblest of all the iron bridges which span the Thames, and of the Hoi- 
born Viaduct, the opening of a railway through the old Thames tunnel, and 
the approaching completion of a new tunnel under the Thames, are results 
which will attest to many future generations the skill, boldness, and industry 
of the engineers of 1869. Now that the bridge which forms part of the 
Holborn Viaduct is reported safe, in spite of the unfortunate cracks on the 
roadway side of its beautiful granite columns, the public will perhaps forget 
a defect the importance of which has been overrated. It is not less to be re- 
gretted by those who are interested in scientific engineering, that so little pre- 
caution should have been taken to secure the columns of this bridge against 
the unequal distribution of stress due to any deviation of the direction of the 
thrust upon them from the vertical, or any deviation of the point of applica- 
tion of the thrust from the centre of the section. That there is a deviation 
in one or other of these respects to an extent which has caused the fissures 
in the columns is obvious. Mr. Muir considers that this is due to a contrac- 
tion of the metal of the centre span from decrease of temperature since the 
erection of the bridge. This is a quite possible cause of the cracking, if it 
can be shown that the amount of the contraction is sufficient. 
Mr. Phipps, in a letter to Engineering , has calculated the thrust on the 
columns, supposing the bridge to consist of three separate arched ribs, and 
finds that the nipping up of the smaller arches and the rise of the neutral 
lines of those arches create a couple tending to rotate the columns towards 
the roadway. This is perfectly correct, but Mr. Phipps has neglected the 
greater couple due to the excess of thrust of the middle arch which tends 
to rotate the columns towards the footways, and must so rotate them before 
the presumed rise of the neutral line could come into play. Further, the 
value found by Mr. Phipps for the bending action on the columns is such 
that the joints ought to be wide open on the footway side, which is not the 
case. If the columns had been loaded by three separate arches, they ought 
to have crushed on the footway side, whereas they have gone on the road- 
way side. The arched ribs are, in fact, bolted together and braced to the 
roadway girders. They are, therefore, more nearly in the condition of a 
continuous girder of three spans than of an arched bridge. Treated as a 
continuous girder, it is not difficult to see that the great excess of length in 
the middle span would tend to give the part of the girder over the columns 
au inclination pointing upwards over the footways, in consequence of the 
deflection of the middle arch ; or if, from the depth of the girder at that 
point, an actual inclination was not produced, still the centre of pressure on 
the bearing would approach the roadway side. 
Whatever the cause of the bending action, the report of the engineers 
appointed to examine the bridge seems to show that they do not consider it 
sufficient to cause ultimate danger to the structure. 
