18 PROCEEDINGS OF THE AMERICAN ACADEMY 



A App 2 



t ''"' 8l Vo (l + $aAp)' 



Hence the maximum flow will occur when A p = — 2 / a, or, as 

 negative pressures are excluded, the function A / t is of a kind con- 

 tinually increasing with A p. 



11. Measurement of excessively high Pressures. — In view of this 

 property of A / 1, it is worth inquiring in how far the transpiration 

 method is available for high pressure measurement, when all other 

 means fail. 



Take for example marine glue, and a tube r \ inch in diameter 

 and 1 inch long. Then the mass m of marine glue which at 18°. 3 

 would exude per hour is given in Table III. 



TABLE III. — Transpiration of Marine Glue, per Hour, at 18°.3 C. 



I = 2.5cm. p = .08 cm. tj = 1.5 (1 + .009J Ap) x 10 9 . 



Ap = 1000, 5000, 10000, 15000, 20000 atra. 



m X 106 = 4232, 4949, 5060, 5096, 5115 grams. 



Thus it appears that whereas a hole T ' ff inch in diameter may be 

 efficiently sealed by marine glue at 18°. 3 C, pressure measurement 

 by aid of the exuding mass is impossible above 10,000 atm., whereas 

 even between 5,000 atm. and 10,000 atm. the method is insensitive. 

 To use a method like the present for very high pressure measure- 

 ment, a substance of smaller pressure coefficient must therefore be 

 sought, if such a one with other necessary qualities, exists. It is 

 with the object of searching for such a body, as well as of finding 

 the maximum of hydrostatic pressure attainable in the laboratory, 

 that I had a tinned screw and socket constructed,* and hope to be 

 able to report the results of my work at an early opportunity. 



* These Proceedings, Vol. XXV. pp. 94, 108, 1890. 



Physical Laboratory U. S. Geological Survey. 



