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The Dominion Observatory—100th Anniversary

Gravity and Seismic Operations

The Dominion Observatory had another mandate — rather than looking up from the Observatory dome, some staff went to the basement to study gravity and seismic activity. The Observatory became part of a growing worldwide network of centres with seismographs to detect earthquakes and to combine the information to determine the location of the seismic activity. The Museum has a couple of early seismographs, but none from the Dominion Observatory.

The Museum does, however, have a couple of examples of gravity sensing apparatus. The oldest is a Mendenhall type gravity apparatus (1987.2133), manufactured by George N. Saegmuller, of Washington, D.C. This apparatus was developed in 1889–90 by T. C. Mendenhall of the United States Coast and Geodetic Survey. The pendulums (1993.0234) used in the gravity apparatus were also made by Saegmuller. Their surfaces are gold plated to minimize corrosion and the absorption of moisture or other contaminants that might change their mass. The first measurements of gravity in Canada were made with this apparatus in August 1902 by Dr Klotz. He travelled across the Pacific as a new underwater cable was laid from Canada to Australia, making gravity measurements along the way in places like Tahiti. But continuous, systematic, operations in Canada did not begin until about 1912. According to Klotz, learning to set up the apparatus in the field required considerable time and skill.

(Fig.22a)
The Mendenhall-type gravity apparatus (CSTM 1987.2133.1) was used across Canada and the Pacific by Otto Klotz.
  (Fig.22b)
Several different pendulums (CSTM 1993.0234) were used in the vacuum within the case to determine the value of “g” by the time they took to make a given number of swings.

The precisely made pendulums were mounted one at a time in the chamber, which was evacuated to minimize temperature changes and protect it from wind. The 0.5 metre long pendulums swing back and forth in exactly 1/2 second at 1 g (the value of gravity at the surface of the Earth at a distance equal to the Earth’s average radius). In locations where the gravity is greater or less than 1 g, the pendulum swings slightly faster or slower respectively. For example, if there is a concentration of mass, such as a deposit of iron or nickel, beneath the apparatus, the pendulum swings faster. The period is measured with a chronometer (1987.2132, Ulysse Nardin, Switzerland). The beautiful and superbly made pendulums used with this apparatus were, with meticulous care, used until the 1970s with more modern apparatus.

  (Fig.23)
The time was measured using a marine chronometer adapted to the task (CSTM 1987.2132). Increased “g” makes the pendulums swing a bit faster.

(Fig.24)
A. H. Miller with the Askania torsion balance (CSTM 1987.2131) and its portable shed that replaced the Medenhall in the late 1920s.
The Museum also has one of two torsion balances used by A. H. Miller of the Geological Survey in the earliest wide-scale gravity surveys conducted in Canada beginning in the 1920s. Such surveys were carried out, among other reasons, to locate bodies of ore for Canada’s growing mining industry. The Museum’s Askania torsion balance (1987.2131, ca 1925–1928) was designed to record observations photographically, as well as permitting visual observations. Two bars are suspended and adjusted to be precisely parallel. Each bar supports two large weights—one on the bar and one suspended from the opposite end. At 1 g the bars remain parallel but if the apparatus is set up where gravity is greater or less than 1 g, the weights on the bars, due to their mutual gravitation, make the bars turn very slightly. A light beam and mirrors magnify the deflection and a direct reading can be made of the value of “g.”