of the Rapid Transit Tunnel, in Brooklyn, in which a great many of the
cast-iron rings were cracked under the crown of the arch, during
construction; but, in spite of this, they sustained, for more than two
years, a loading which, according to Mr. Goodrich, was continually
increasing. In other words, the cracked arch sustained a greater loading
than that which cracked the plates during construction, according to his
theory, as noted in the following quotation:

"But it should be equally conceded by the advocates of the
existence of such action that changes in humidity, due to moving
water, vibration, and appreciable viscosity, etc., will invariably
destroy this action in time."

As to the correctness of this theory Mr. Goodrich would probably have
great difficulty in convincing naturalists, who are aware that many
animals live in enlarged burrows the stability of which is dependent on
the arching action of the earth; in fact, many of these burrows have
entrances under water. He would also have some difficulty in convincing
those experienced miners who, after a cave-in, always wait until the
ground has settled and compacted itself before tunneling, usually with
apparent safety, over the scene of the cave-in.

The writer quotes as follows from Mr. Goodrich's discussion:

"In any case, no arch action can be brought into play until a
certain amount of settlement has taken place so as to bring the
particles into closer contact, and in such a way that the internal
stresses are practically those only of compression, and the
shearing stresses are within the limits possible for the material
in question."