SECTION OF COVERED-EYED MEDUS2. 81 
down. The first wave which 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 wave, 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 contraction-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 
