394 
PHILOSOPHICAL SOCIETY OF WASHINGTON. 
held the relation was a numerical one rather than a geometrical 
one. Mr. Gore concurred in this view. 
Mr. Gilbert presented informally the geophysical problem 
of the pressure of a glacier on its bed at a point below the sur¬ 
face of the ocean, and the contradictory solutions that had been 
given. This was. discussed by Messrs. Winston, Wead, Spill¬ 
man, Marvin, Gore, and Paul. 
Mr. J. H. Gore read a paper on The ambiguity of the double 
sign ± occurring in the extraction of roots. He pointed out 
that ordinarily we determine by experience which of these is the 
true one in a specific case; but in cases outside of experience we 
have no criterion to guide our choice. [Not published.! 
Mr. C. K. Wead then spoke on The theory of some peculiar 
musical instruments in the National Museum. The instruments 
discussed included some globular four-hole whistles that follow 
the resonator law, and various primitive flutes that have an equal 
linear division. Both classes were united under a broad generic 
law applicable to instruments rather than to scales, as is the 
case with our law of scale relations. [A summary of a paper in 
the Eeport of the U. S. National Museum for 1900, p. 417.] 
Mr. XJpham exhibited and played upon several of the instru¬ 
ments that had been discussed. 
551st Meeting. April 12, 1902. 
Vice-President Gore in the chair. 
One hundred persons present. 
The election and qualification of Messrs. S. W. Stratton and 
W. J. Spillman were announced. 
Mr. G. A. Bobrick spoke by invitation on Liquid air, its his¬ 
tory, manufacture, and uses. The classical experiments were 
shown and the physical properties stated. The methods used 
during a century for liquefying gases are three: the bent tube 
(Davy), the cascade or closed double cycle (as by Pictet), and 
the self-intensive or regenerative system. Lantern slides illus¬ 
trated this last. In practice this yields a pound of liquid air per 
