seconds for 90 minutes on the one occasion when the 2-element array was used. 

 The instrument had a natural period of 7 seconds and was critically damped. 

 Although this gravimeter was inherently more sensitive to shorter periods than 

 was the LaCoste-Romberg #22, there did not appear to be any energy with periods 

 less than 10 seconds. 



A general discussion of the physical construction and theory of gravi- 

 meters can be found in any text on exploration geophysics, for example. Nettle- 

 ton (I9i^0). 



d. Microbarograph 



Recordings of micropressiire variations in the period range 10 to 100 

 seconds were desired for correlation with observed ocean waves. A T-21 micro- 

 barograph using pressure transducer #7156 and amplifier #1^779 was used to 

 automatically record the verticeO. micropressure variations. This instrument 

 was built by the U.S. Kavy Electronics Laboratory for the Air Force Cambridge 

 Research Center. 



The transducer is basically a condenser microphone composed of two 

 enclosed volumes separated by a thin aluminum diaphragm. The condenser is 

 formed by the diaphragm and a backing plate with 0.005- inch separation. Each 

 enclosed volume is vented to the atmosphere, but the time constant of each is 

 different. Thus, for a given rate of atmospheric pressure change, a pressure 

 difference exists across the diaphragm. Frequency response of the transducer 

 is determined by choice of acoustic vents in the enclosed volumes. The re- 

 sponse curve for the instrument used in this work is shown in fig\are 3« 



PERIOD (SECONDS) 



Microbarograph Response Curve 



