160 



the higher tides which are also present. It is difficult, but not impossible, to 

 obtain sufficiently long time constants by means of electric filters. The difficul- 

 ty has to do with the leakage of high-capacity condensors. Pneumatic devices 

 work rather well. The "RC-time" of such pneumatic devices, regardless of how 

 the capillaries, air and liquid volumes are connected, and of other plumbing de- 

 tails, varies as r^ r"^ , where r and r are the radii of the pipes and capillar- 

 ies used (Munk, 1948). We do not generally desire r to be much larger than a 

 foot, nor r^ to be much smaller than a millimeter. These physical limitations 



place the pneumatic methods in the indicated range of periods. 



One difficulty with pneumatic devices is that they become involuntary 

 temperature recorders. The painful fact is that at atmospheric pressure a 1°C 

 temperature change in an enclosed air volume will induce a pressure change 

 equivalent to that exerted by 3 cm of water. At a depth of 300 feet a change by 

 only 0.1°C corresponds to 3 cm of water. For this reason the tsunami record- 

 ers at the Scripps Institution have been designed to be temperature compensated, 

 to a first approximation; in addition, they have been placed into a constant-tem- 

 perature box, and the temperature is recorded for additional control. For peri- 

 ods much longer than 10"* seconds the "temperature noise" becomes so large 

 that it would seem better to abandon pneumatic devices altogether. 



One obvious method is to subtract the predicted tide. This method 

 should permit the detection of large storm surges with periods between 10'* and 

 105 seconds. Weekly and monthly tide averages have been used, but these con- 

 tain a rather large tidal residue. Mr. Gordon Groves is devising a more accu- 

 rate method which appears to be well adapted for wave periods in excess of 10^ 

 seconds. The principle is to take hourly water level readings from tide gauges, 

 and to combine these readings in a special manner so as to eliminate the semi- 

 diurnal and diurnal tides, but to give nearly full response to other periods. The 

 numerical filter has the following two advantages: it discriminates sharply 

 against the precise periods of the various tidal constituents, making good use of 

 the fact that these periods are known with great accuracy from astronomic ob- 

 servations; and it has the stability that is esential for the study of long-term 

 changes. The calculations can be performed by means of punch cards. In this 

 manner it is hoped that tide gauge records may become a more accurate and ac- 

 cessible tool to oceanographic studies than they have been in the past. It should 

 be noted that the tidal constants are now computed by punchcard methods, so that 

 hourly values are available in suitable form. However, the hourly values are 

 punched manually, and the original recording is still on paper in pencil, much 

 like in Kelvin's day. Is it not possible that the cards be punched automatically 

 at the tide gauges? 



For longer-than-annual changes in sea level, the principal difficulty with 

 tide gauge records is that they depend more on the up and down movements of 

 continents than on eustatic changes in sea level. In view of this high "geologic 

 noise" I am not certain whether any reduction of tide records, no matter how 

 painstaking, can give any convincing evidence regarding changes in sea level. 

 Perhaps these very slow oscillations in sea level can best be measured by as- 

 tronomic means. Changes in sea level due to melting of ice at high latitudes 

 will affect the earth's inertia, and hence its rate of rotation; furthermore, be- 

 cause of the asymmetry of land and sea, the position of the axis of rotation is al- 

 so changed (Munk and Revelle, in press). These changes can be determined by 

 measuremients of astronomic longitude and latitude, respectively. With the ac- 

 curacy now claimed for both of these measurements it should be possible to de- 

 tect eustatic changes in sea level by only 1 cm, provided the effect of such 

 changes is not obscured by other, larger effects. 



