The ultimate pendulum clock, indeed the ultimate mechanical clock of any kind, was invented by a british engineer, william shortt. the first was installed in the royal observatory in edinburgh in 1921. the shortt clock had two pendulums, primary and secondary. the primary pendulum swung freely in a vacuum chamber. its only job was to synchronize the swing of the secondary pendulum, which was housed in a neighboring cabinet and drove the time-indicating mechanism. every 30 seconds the secondary pendulum sent an electrical signal to give a nudge to the primary pendulum. in return, via an elaborate electromechanical linkage, the primary pendulum ensured that the secondary pendulum never got out of step. shortt clocks were standard provision in astronomical observatories of the 19205 and 19305, and are credited with keeping time to better than two milliseconds in a day. many were on record as losing or gaining no more than one second in a year—a stability of one part in 30 million. the first indications of seasonal variations in the earth's rotation were gleaned by the use of shortt clocks. in 1984 pierre boucheron carried out a study of a shortt clock which had survived in the basement of the united states naval observatory since 1932. after replacing the electromechanical linkage with modern optical sensing equipment, he measured the shortt clock's rate against the observatory's atomic clocks for a month. he found that it was stable to 200 microseconds a day over this period, equivalent to two to three parts in a billion. what is more, the data also revealed that the clock was responding to the slight tidal distortion of the earth due to the gravitational pull of the moon and the sun. in addition to causing the familiar ocean tides, both the sun and the moon raise tides in the solid body of the earth. the effect is to raise and lower the surface of the earth by about 30 centimeters. since the acceleration due to gravity depends on distance from the center of the earth, this slight tidal movement affects the period of swing of a pendulum. in each case the cycle of the tides caused the clock to gain or lose up to 140 microseconds.

1. the passage most strongly suggests that its author would agree with which of the following statements about clocks? a) before 1921 no one had designed a clock that used electricity to aid in its timekeeping functions. b) atomic clocks depend on the operation of mechanisms that were invented by william shortt and first used in the shortt clock. c) no type of clock that keeps time more stably and accurately than a shortt clock relies fundamentally on the operation of a pendulum. d) subtle changes in the earth's rotation slightly reduce the accuracy of all clocks used in observatories after 1921.e) at least some mechanical clocks that do not have pendulums are almost identical to shortt clocks in their mode of operation.2. according to the passage, the use of shortt clocks led to the discovery thata) optical sensing equipment can be used effectively in time-keeping systemsb) atomic clocks can be used in place of pendulum clocks in observatoriesc) tides occur in solid ground as well as in oceans d) the earth's rotation varies from one time of year to anothere) pendulums can be synchronized with one another electronically3. the passage most strongly suggests that the study described in the third paragraph would not have been possible in the absence ofa) accurate information regarding the times at which high and low ocean tides occurred at various locations during 1984b) comparative data regarding the use of shortt clocks in observatories between 1921 and 1932 c) a non-shortt clock that was known to keep time extremely precisely and reliablyd) an innovative electric-power source that was not available in the 1920s and 1930s e) optical data-transmission devices to communicate between the u. s. naval observatory and other research facilitiesspoiler: : : 4. the passage most strongly suggests that which of the following is true of the chamber in which a shortt clock's primary pendulum was housed? a) it contained elaborate mechanisms that were attached to, and moved by, the pendulum. b) it was firmly sealed during normal operation of the clock. c) it was at least partly transparent so as to allow for certain types of visual data output. d) it housed both the primary pendulum and another pendulum. e) it contained a transmitter that was activated at irregular intervals to send a signal to the secondary pendulum.