EVOLUTION OF QUARTZ CRYSTAL CLOCK 569 



portable equipment. This original crystal chronometer has been on several 

 gravity-measuring expeditions and is still in active service, having been used 

 again under Dr. Ewing's direction during the summer of 1947. 



Gravity determinations at sea are made by measuring the rate of a special 

 triple pendulum that was invented by F. A. Vening Meinesz especially for use 

 in unsteady environments^^^ Previously, the standard of rate had been 

 the usual ship's chronometers, but Ewing found the crystal chronometer to 

 be an improvement for his purposes, saying in part: 'This chronometer is not 

 thermostatted, and temperatures in a submarine change greatly during a 

 dive. No elaborate control over battery voltages was used. The cruise 

 started in the tropics and ended in Philadelphia in mid-winter. It is highly 

 significant that the interval between NAA-time and the chronometer-time 

 never exceeded- 0.6 second during the six-week's cruise and that the 

 variation in this inter\'al is very regular. The crystal chronometer has 

 reduced errors in gravity-measurements at sea, due to the rate of the chro- 

 nometer, to the point where they are negligible." 



Some years previous to the construction of the crystal chronometer, a self- 

 contained quartz clock was made to illustrate the possibility of a compact 

 assembly, but it was not sufficiently portable for the submarine expedition. 

 This earlier clock was regulated by a quartz sphere such as used by 'crystal 

 gazers'. The frequency of the sphere was not adjusted, but its natural 

 frequency, which happened to be 33212, was adopted to operate a mean-time 

 dial by the choice of a suitable gear train. Since that time much more 

 compact assemblies have been built using more suitable crystals for control. 



The stable properties of the quartz clock have been useful in a number of 

 cases requiring precise synchronization. Perhaps the most noteworthy 

 among these is the application to Long Range Navigation known as LORAN. 

 In this application, pairs of transmitting stations, usually on shore and sep- 

 arated by accurately known distances, send out distinctive signals in syn- 

 chronism. The tune interv^al between these signals, as received by a ship, 

 identifies the locus of all the points corresponding to that time interval. 

 The set of curves corresponding to all feasible time intervals defines one of 

 the coordinates in a two-coordinate system. The other coordinate is pro- 

 vided in identical manner by another pair of shore transmitters (which 

 may have one station in common with the first pair). The resulting coor- 

 dinate system consists of two families of intersecting hyperbolas. From the 

 geometry of these curves, and the constants of the signals, the complete 

 figure bounded by the ship and the transmitters can be determined readily. 



The need for stability is evident from the fact that the relation between 

 time error and location error is roughly 5 microseconds per mile. In some 

 cases, location within a mile is highly desirable even at considerable dis- 



