SECTION OF COVERED-EYED MEDUS.E. 81 



down. The first wave wliich effects a passage 

 appears to have nearly all its force expended 

 in overcoming the barrier, the residue being only 

 sufficient to cause a very feeble, and sometimes 

 almost imperceptible, contraction of the umbrella. 

 The next Avave, however, passes across the barrier 

 with more facility, so that the resulting contraction 

 of the umbrella is more decided. The third wave, 

 again, causes a still more pronounced contraction of 

 the umbrella ; and so on with all succeeding waves, 

 until every trace of the previous blocking has 

 disappeared. When this is the case, it generally 

 happens that the strip will again admit of being 

 elongated for a short distance before a blocking of 

 the contract! on- waves again supervenes. Sometimes 

 it will be found that this second blockage will also 

 be overcome, and that the strip will then admit of 

 being still further elongated without the passage 

 of the waves being obstructed ; and so on occa- 

 sionally for three or four stages. 



The same series of phenomena may be shown in 

 another way. If a contractile strip of tolerable 

 length be obtained, with the waves passing freely 

 from one end to the other, and if a series of parallel 

 and equidistant cuts be made along one side of the 

 strip, in a direction at right angles to the length, 

 and each cut extending two-thirds of the breadth 

 of the strip, the chances are in favour of the con- 

 traction-waves being wholly unaffected by the sec- 

 tions, however numerous these may be. But now, if 

 another series of parallel and equidistant cuts of the 

 same length as the first ones, and alternating with 



