THEORETICAL EFFECTS OF PRESSURE WAVES. 221 



has been divided into squares, and in each square the pressure has been entered in agreement 

 with the isobars. The waves which are to be impressed on this pressure distribution travel 

 from Framheim to Cape Adare, i.e., in the direction of the line of squares a, b, c, ... n, o. 

 The wave fronts are at right angles to this line. Values will be given to the letters 

 a, b, c, etc., according to the pressure of the wave which is travelling from a to o. As 

 the pressure due to the wave is the same along the whole wave front the values of a, b, c, 

 etc., must be added to the pressures in the line of squares to which the letter is attached. 

 Thus the value of i determined by the position of the wave has to be added to the pressure 

 entered in each of the squares along the line E to i. 



The wave to be investigated may vary in two particulars, (a) in amplitude and (6) in 

 the difference in phase %^ath which it arrives at the three stations. A large number of waves 

 having different amplitudes and phases have been investigated, and it is found that the 

 result is not affected in principle, but only in detail by changing these variables. From 

 these waves the one has been chosen for discussion here which gives the best general 

 result. 



This wave is given at Cape Evans by dp =-3" cos 9 and it travels from Framheim 

 to Cape Adare in a quarter of its period. So that the pressure changes at Framheim are 

 dp=-3" cos (61+45°) and at Cape Adare dp=-3" cos (6-45°). 



In figure 6.56 the wave to be investigated is shown in the same way as the pressure 

 curves on the plates in Volume II. The abscissa represent equal intervals of time, but as we 

 are not concerned with absolute time, which involves the rate of travel of the waves, but 

 only with time in so far as it affects the phase of the wave when it arrives at the different 

 stations, the phase of the wave at each interval at Cape Evans is given instead of the time. 

 The wave arrives at each station an eighth of a period after it left the last. Thus 

 if the phase at Cape Evans is e, it is 6+45° at Framheim and 9-45° at Cape 

 Adare. 



The most important curve on the plates in Volume II was found to be the one giving 

 the difference in pressure between Cape Evans and Framheim, a similar curve therefore has 

 been added to figure 656. 



We shall consider the effect of the wave at eight intervals during its passage commencing 

 from the epoch when the maximum of the wave has just reached Cape Adare. The posi- 

 tions of the wave at each of the epochs considered are shown in figure 656 by the vertical 

 lines at 



e = io", 93°, 135°, 180°, 225°, 270°, 315° and 360°. 



For each position of the wave two diagrams are given. In the first the pressure which 

 results by the simple addition of the pressure wave to the normal pressure is shown. In the 

 second an attempt has been made to allow for the alteration in the pressure which the 

 air motion would produce, taking into account the inertia of the air and the obstacles to 

 its motion. Thus for each position of the wave there will be two diagrams similar to figures 

 64a and 646, which are the corresponding diagrams for the conditions in the absence of the 

 pressure waves. 



