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SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 62 



adopted at Gottingen to measure the resistance of hulls, etc., held 

 obliquely to the current. Prandtl's differential pressure gauge, con- 

 sisting of inverted cups suspended from opposite arms of a balance, 



Fig. 2. — Prandtl's Suspension for Measuring Head Resistance 



Fig. 3. — Prandtl's Manometric Balance 



and dipping into a liquid, is like the one devised and used by me 

 early in 1902, and found capable of measuring differential pressures 

 truly to one millionth of an atmosphere, or less. This gauge was 



