74 



WORK OF THE CARNEGIE AND SUGGESTIONS FOR FUTURE SCIENTIFIC CRUISES 



in which ci is the temperature constant of first original 

 pendulum, di is the density constant of first original 

 pendulum, T is the period of the fictitious pendulum, and 

 r is the daily rate in seconds of the chronometer used. 

 The values of 6T are the quantities which must be sub- 

 tracted from the observed period of the fictitious pendu- 

 lum in order to obtain the reduced period. The reduced 

 period is the same as Tj, the period of the first original 

 pendulum. 



The Appar atu s 



The apparatus contains three principal quarter- 

 meter pendulums, which are nearly isochronous and 

 which swing in the same vertical plane. To facilitate 

 description, the outer ones are designated as numbers 1 

 and 3, and the middle one as number 2. Their knife 

 edges are about thirteen- centimeters apart. By means 

 of mirrors the following angles of elongation are record- 

 ed: (5i - 02), (03 - 62), and 82- The first two of these 

 are the angles of elongation of two fictitious pendulums. 

 The last provides the data to determine the corrections 

 to the periods of the two fictitious pendulums which re- 

 duce them to Ti and T3, respectively. During the rou- 

 tine gravity observations the two outer pendulums are 

 swung with equal amplitudes and opposite phases, where- 

 as the middle pendulum is given as little amplitude as 

 possible. It should be emphasized that the values of 

 gravity are derived from each of the two fictitious pen- 

 dulums. 



The datum for recording 92 is a critically damped 

 pendulum in the same plane as the other three. A sec- 

 ond critically damped pendulum, in a plane at right- 

 angles, records the tilt of the swinging plane. 



Inside the apparatus there is also a dummy pendu- 

 lum which contains a thermometer. There is also ahair 

 hygrometer. Provision is made for mechanically lifting 

 the pendulums off their knife edges and for firmly 

 clutching and locking them when they are not in use, so 

 they cannot be damaged during any rough weather at sea. 

 Mechanical means also are provided for lowering the 

 pendulums slowly so there will be no damage done to the 

 agate knife edges or to the agate planes on which they 

 rest. Further control is provided for giving the pendu- 

 lums known amplitudes and phase relations at the begin- 

 ning of an observation. These operations are carried 

 out from the outside of a double-walled he at -insulated 

 metal box, which contains the pendulums. Figure 2 

 shows the outward appearance of this part of the appara- 

 tus. The controls just described appear below the name 

 plate on the front. Figure 3 is a top view of the appara- 

 tus after it has been removed from the box shown in fig- 

 ure 2. It shows the arrangement of the mirrors, prism.s, 

 and lenses for recording the pendulum movements. The 

 top part of the apparatus, as shown in figure 1, contains 

 the photographic paper for recording the pendulum 

 movements. It also contains the source of light, the 

 shutter mechanism which is operated by the two chro- 

 nometers, the controls for altering the speed of the 

 photographic paper, and a small window through which 

 the movements of the light images on the paper may be 

 observed during an observation without interrupting the 

 recording. 



The Record 



Reading from bottom to top, figure 4 shows, on a 

 rsduced scale, three records obtained respectively in 

 San Francisco, Honolulu, and Pago Pago (American 

 Samoa). The actual width of each record is about 12 cm, 

 and its duration is about thirty minutes. On each record 

 the two upper curves, which have about the same width, 

 are derived from the oscillations of the two fictitious 

 pendulums, whereas the lower curve is derived from the 

 oscillations of the middle pendulum alone. Its variable 

 amplitude indicates how it is affected by the ship's 

 movements. The regular amplitudes of the two fictitious 

 pendulums show that these are quite unaffected by the 

 ship's movements. 



Superimposed on the trace of each fictitious pendu- 

 lum are four sine curves. Two of these, which are 180° 

 out of phase, are produced by a mean-time chronometer, 

 whereas the other two are produced by a sidereal chro- 

 nometer. Each chronometer, by means of an electrical 

 circuit, causes a shutter to pass every half second in 

 front of the beam of light, and interrupts the recording 

 for about 0.01 second, thus producing the series of white 

 marks which form the phase-lag curves. The period of 

 each fictitious pendulum is obtained independently from 

 each of the two chronometers by carefully measuring 

 the time interval in which the phase-lag curve goes 

 through a known number of cycles. In this way the un- 

 corrected period of each fictitious pendulum can be de- 

 termined from good records with an error not greater 

 than about 5 x 10"7 second. 



From the bottom curve on each record are obtained 

 the corrections to each of the two fictitious pendulum 

 periods, for deviation from isochronism, and also for 

 reduction to infinitesimal amplitude. To facilitate the 

 necessary measurements for these corrections, the 

 shutter mechanism is changed so that the beam of light 

 is interrupted for alternate half seconds, and to save 

 paper and not make the record unnecessarily long, the 

 speed of the paper is reduced. This explains the change 

 in appearance of the middle part of each record. The 

 observations are made by giving the two outer pendulums 

 equal amplitudes and opposite phases, whereas the mid- 

 dle pendulum is given no amplitude, though, of course, 

 its amplitude is soon increased by the ship's movements. 



Behavior of Apparatus on Carnegie 



No particular difficulty v/as 3xperienced in obtaining 

 gravity records on board the Carnegie while she was in 

 port at San Francisco, Honolulu, and Pago Pago (Ameri- 

 can Samoa), although the amplitude of the middle pendu- 

 lum in all but one of the records obtained at San Francisco 

 always got quite large considering the small movements 

 of the shio. This doubtless was owing, not so much to 

 the amplitude of the ship's movements, as to the irregu- 

 lar movements caused by the "rubbing" of the ship 

 against the pier. 



Several attempts were made to obtain gravity deter- 

 minations at sea during the seventeen-day voyage from 

 San Francisco to Honolulu, and again during the forty- 

 seven days from Honolulu to Pago Pago (American Sa- 

 moa). Most of these were unsuccessful because the 

 amplitudes of the pendulums got too large and in a few 

 instances there was actual slipping of the knife edges. 

 Figure 5 is a reproduction of three records obtained at 



