Transit instrument at the Thames Tunnel shaft

The transit instrument at the Thames Tunnel shaft, used during the survey of January 1827, consists of a telescope mounted to swing only in the vertical plane. Alignment must be absolute. A deviation of a single minute at the surface results in a catastrophic error of feet when the two headings meet beneath the riverbed. The instrument is a hard record of the man who leveled it. If the surveyor possesses a nervous disposition or a tendency toward haste, the brass screws will show the scarring of forced adjustments. A calm hand leaves the lacquer intact. The tool reveals the builder’s tolerance for ambiguity.

The brickwork follows the line set by the transit. The shield moves forward in four-inch increments. The pressure of the saturated silt above the arch exerts a constant downward force of several thousand tons. The mortar must set before the tide turns, or the salt water will leach the lime and collapse the face. I found the transit knocked from its mounting after the first breach. The tripod legs were splintered by the rush of the river. The failure mode was not the glass or the brass, but the stability of the ground upon which the measurement was based. The instrument cannot account for the liquefaction of the soil it stands on.

A builder who trusts his instrument without checking the plumb-line every hour is a man who invites the river into his lungs. The transit shows that I demand a precision that the environment refuses to give. The tool is rigid; the mud is fluid. The conflict between the two is where the engineering actually happens. I have recalibrated the sights to account for the vibration of the steam pumps. The new mounting is bolted directly into the iron kerb of the shaft. The measurement is now independent of the surface tremors. I will go down the shaft at midnight to verify the heading myself.