r 
-428 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 13, No. 19 
by graphical integration, m having previously been determined by 
another graphical integration according to equation (8). The pres- 
sures at various depths are plotted in Fig. 5. At the center the 
pressure is 3.18 millions of megabars, remarkably close to the value 
obtained from Laplace’s law (3.08 million megabars) when the surface 
density is 2.7. 
SUMMARY 
For the density and composition of the Earth at various depths 
there is here proposed a distribution which takes into account the 
density change due to compression alone. When it is noted that a 
pressure of 1,000,000 megabars is reached at a depth of less than 
2400 km, it is evident that the reduction in volume under such a 
pressure is a factor not to be neglected. By the use of earthquake 
data a quantitative estimate is given of the density change due to 
compression of a homogeneous material at various depths—or of that 
part of the density change due to compression alone in the case of a 
variable composition. The present distribution, moreover, reconciles 
the continuity of the velocity depth curves with the difference in 
velocity in metallic iron and in basic silicate. 
Of the four zones described two are sensibly constant in compo- 
sition but not of constant density (the central core of nickel-iron and 
the peridotite shell immediately below the surface layer), and two are 
of variable composition (the surface layer and the pallasite fringe 
surrounding the metallic core). 
The distribution here suggested is at best a rough approximation, 
but it seems to be the simplest possible arrangement consistent with 
the physical, seismologic and astronomic data. 
In a paper by Gutenberg (Phys. 94: 296-9. 1923) which has just come to our atten- 
tion, there is given a density-depth curve which like ours consists of four parts. By 
assuming the core to be of constant density 2.3 times that of the next layer (also of 
constant density), Gutenberg calculates that the density of the “Mantel,” extending 
from 60 km to 1200 km depth, varies from 3} to 4}. This estimate of the density 
change in the outer parts of the Earth is strikingly like our estimate obtained 
directly from compressibility and involving assumptions quite different from those of 
Gutenberg. 
BOTANY.—Note on plants collected in tropical America. H. Prrrtmr. 
Between October 1887 and the present time, I have collected about 
18,000 plants in Mexico, Guatemala, Salvador, Honduras, Costa 
Rica, Panama, Colombia and Venezuela. These plants have been 
numbered in two series, and, as the numbers of the one series are 
