34 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 62 



(4) Holes should be symmetrically distributed on the cylindrical 

 portion of the tube. 



(5) Any new type of tube should be calibrated against a standard. 



The Inclined Manometer 



Granted that a Pitot tube is at hand which will correctly transmit 

 the static pressure, measurements of velocity are no better than the 

 manometer used. The ordinary U-tube filled with water, gasoline, 

 alcohol, or other light liquid shows a head of less than i inch for ordi- 

 nary velocities. To obtain the velocity head within i per cent, it 

 would be necessary to read the displacement of the meniscus to within 

 o.oi inch. This is hardly practicable, and various devices are used 

 to magnify the reading. It is at once apparent that if the U-tube be 

 canted at an angle of i in 20, a i-inch head of liquid corresponds 

 to 20 inches on the scale. With an inclined tube, the diameter of 

 the tube must be kept small in order to obtain a good meniscus for 

 reading. On the other hand, in any gage in which the imperfections 

 in the glass produce changes in capillarity, the liquid sticks at some 

 places. A large tube tends to reduce this source of error. The 

 American Blower Company recommend an inclined U-tube filled 

 with gasoline for use with the Pitot tube. This type involves the 

 simultaneous reading of the meniscus level in each leg of the tube, a 

 somewhat difficult feat in an unsteady current. 



The German " Krell " manometer is filled with alcohol colored to 

 give a visible meniscus. One leg of the U-tube is an inclined glass 

 tube, and the other is a reservoir bottle whose section is some 400 

 times the section of the tube. Hence, as liquid rises in the glass tube 

 the depression in the reservoir is unimportant. Only one meniscus 

 level then need be read. 



An inclined tube manometer on the Krell principle was con- 

 structed, and an investigation made of its errors by comparing it with 

 a Chattock manometer known to be nearly correct. This alcohol 

 manometer is shown in figure 8. It is seen to include a reservoir R 

 mounted on a hinged plate with leveling screw. By means of the 

 latter, the liquid in the tube is brought to the zero of the scale at the 

 beginning of a test, thus making a zero correction unnecessary. The 

 glass tube T is likewise mounted on a brass plate pivoted at the knife 

 edge K, and adjusted in pitch by the screw 5". To the brass plate are 

 attached permanently two small machinists' spirit levels L^ and Lo, 

 set at 3 degrees and 6 degrees to the axis of the tube. The corre- 

 sponding pitch is roughly i in 10 and i in 20. For a low velocity 



