194 



PRESSURE. 



There can be little doubt from this inspection that the same surges are recognisable on 

 all three curves. But it might reasonably be asked why the maxima and minima of the 

 surges do not coincide more closely on the three curves, for there are often long periods 

 between the appearance of the same maxima and minima at different stations. The reason 

 is not difficult to find. The true surge probably does affect each station at approximately 

 the same time, but superposed upon the long period surges are numerous pressure changes 

 due to local conditions. We attempt to eliminate these by taking the ten-day means, but 

 it is impossible to eliminate them entirely. When a surge is only feebly marked at a station, 

 the position of the actual maximum or minimum is often detei mined accidentally by the 

 minor irregularities due to these local pressure changes. We have already seen how the local 

 disturbance at Snow Hill about the middle of March completely masked the maximum of a 

 small surge which should have then occurred, and attention has been drawn to the position 

 of the minimum at the end of June, which was decided at Snow Hill and the Gauss Station 

 by small local inequalities of pressure on the very flat trough of a surge. It is probable 

 that in every case where the surges appear out of phase at the different stations the cause 

 is to be found in the local pressure variations. 



We will now compare the intensity of the surges at the three stations. 



For this purpose only the maxima and minima which are marked on all three curves 

 by a corresponding sign are entered in the following table. This procedure automatically 

 excludes all the doubtful surges. 



Table 113. 



