[Illustration: Figure 31.--EXCAVATION IN FRONT OF SHIELD, Tower
Subway. This was possible because of the stiffness of the clay
encountered. MHT model--front of model shown in fig. 29.
(Smithsonian photo 49260-A.)]
Under this thin pretense of legal authorization, the sub-rosa
excavation began from the basement of a clothing store on Warren
Street near Broadway. The 8-foot-diameter tunnel ran eastward a short
distance, made a 90-degree turn, and thence southward under Broadway
to stop a block away under the south side of Murray Street. The total
distance was about 312 feet. Work was carried on at night in total
secrecy, the actual tunneling taking 58 nights. At the Warren Street
terminal, a waiting room was excavated and a large Roots blower
installed for propulsion of the single passenger car. The plan was
similar to that used with the model in 1867: the cylindrical car
fitted the circular tunnel with only slight circumferential clearance.
The blower created a plenum within the waiting room and tunnel area
behind the car of about 0.25 pounds per square inch, resulting in a
thrust on the car of almost a ton, not accounting for blowby. The car
was thus blown along its course, and was returned by reversing the
blower's suction and discharge ducts to produce an equivalent vacuum
within the tunnel.
[Illustration: Figure 32.--INTERIOR OF COMPLETED TOWER SUBWAY.
(THORNBURY, _Old and New London, 1887, vol. 1, p. 126_.)]
The system opened in February of 1870 and remained in operation for
about a year. Beach was ultimately subdued by the hostile influences
of Boss Tweed, and the project was completely abandoned. Within a very
few more years the first commercially operated elevated line was
built, but the subway did not achieve legitimate status in New York
until the opening of the Interborough line in 1904. Ironically, its
route traversed Broadway for almost the length of the island.
[Illustration: Figure 33.--VERTICAL SECTION through the Greathead
shield used at the Tower Subway, 1869. The first one-piece shield of
circular section. (COPPERTHWAITE, _Tunnel Shields and the Use of
Compressed Air in Subaqueous Works_.)]
The Beach shield operated with perfect success in this brief trial,
although the loose sandy soil encountered was admittedly not a severe
test of its qualities. No diaphragm was used; instead a series of 8
horizontal shelves with sharpened leading edges extended across the
front opening of the shield. The outstanding feature of the machine
was the substitution for the propelling screws used by Brunel and
Greathead of 18 hydraulic rams, set around its circumference. These
were fed by a single hand-operated pump, seen in the center of figure
34. By this means the course of the shield's forward movement could be
controlled with a convenience and precision not attainable with
screws. Vertical and horizontal deflection was achieved by throttling
the supply of water to certain of the rams, which could be
individually controlled, causing greater pressure on one portion of
the shield than another. This system has not changed in the ensuing
time, except, of course, in the substitution of mechanically produced
hydraulic pressure for hand.
[Illustration: Figure 34.--BEACH'S Broadway Subway. Advancing the
shield by hydraulic rams, 1869. MHT model--1-1/2" scale. (Smithsonian
photo 49260-E.)]
[Illustration: Figure 35.--VERTICAL SECTION through the Beach shield
used on the Broadway Subway, showing the horizontal shelves (C), iron
cutting ring (B), hydraulic rams (D), hydraulic pump (F), and rear
protective skirt (H). (_Scientific American_, March 5, 1870.)]
Unlike the driving of the Tower Subway, no excavation was done in
front of the shield. Rather, the shield was forced by the rams into
the soil for the length of their stroke, the material which entered
being supported by the shelves. This was removed from the shelves and
hauled off. The ram plungers then were withdrawn and a 16-inch length
of the permanent lining built up within the shelter of the shield's
tail ring. Against this, the rams bore for the next advance. Masonry
lining was used in the straight section; cast-iron in the curved. The
juncture is shown in the model.
[Illustration: Figure 36.--INTERIOR of Beach Subway showing iron
lining on curved section and the pneumatically powered passenger car.
View from waiting room. (_Scientific American_, March 5, 1870.)]
Enlarged versions of the Beach shield were used in a few tunnels in
the Midwest in the early 1870's, but from then until 1886 the shield
method, for no clear reason, again entered a period of disuse finding
no application on either side of the Atlantic despite its virtually
unqualified proof at the hands of Greathead and Beach. Little precise
information remains on this work. The Beach system of pneumatic
transit is described fully in a well-illustrated booklet published
by him in January 1868, in which the American Institute model is
shown, and many projected systems of pneumatic propulsion as well
as of subterranean and subaqueous tunneling described. Beach again
(presumably) is author of the sole contemporary account of the
Broadway Subway, which appeared in _Scientific American_ following its
opening early in 1870. Included are good views of the tunnel and car,
of the shield in operation, and, most important, a vertical sectional
view through the shield (fig. 35).
It is interesting to note that optical surveys for maintenance of the
course apparently were not used. The article illustrated and described
the driving each night of a jointed iron rod up through the tunnel
roof to the street, twenty or so feet above, for "testing the
position."