14 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 62 



identical in the two gages, but the glass work was purposely altered. 

 The tip of the tube at B was ground to a knife edge in one gage and 

 in the other ground off square. One tube was .20 inch in diameter 

 and the other .15 inch in diameter. 



The two gages were connected to the same static pressure and gave 

 readings identical to 0.25 per cent. It was found that the gage in no 

 way is affected by minor variations in workmanship. 



In the gage with the ground knife edge tip it was found that the 

 bubble did not break so readily as in the gage with the square tip. It 

 was suggested by Professor Gill that the tenacity could further be 

 increased by coating the outside of the tube below the bubble with 

 paraffin. This was tried and was found to be of great assistance. A 

 height of bubble from three to four times the diameter of the tube at 

 its base could be allowed without rupture. The reason for this is to 

 be found in the fact that castor oil sticks very tight to glass but will 

 not stick to paraffin. By the use of this wax the bubble could not 

 creep over the edge of the tube and so slide down it causing a break. 

 However, any large excursion of the bubble is to be avoided as tend- 

 ing to cause a slight change in the zero reading. In all tests the zero 

 should be taken at intervals. The effect of the paraffin on the tip 

 could not be detected in the readings of the gage. 



The consistency of the gage readings with these various alterations 

 in the base of the bubble as well as in the size of the bulbs and con- 

 necting tubes gives great confidence in this type of gage. It was not 

 possible to calibrate this gage experimentally because there was no 

 other gage available to measure it against which was equally sensitive. 

 However, we have Professor Chattock, Dr. Stanton, and the National 

 Physical Laboratory as authority for the calibration of the gage by 

 calculation from the dimensions of its parts, and the density of the 

 liquid. It may be noted that the density of the oil used has no effect on 

 the principle of the gage and is not considered. 



For the calculation of tilt, it is then necessary to measure the dis- 

 tance between the centers of the bulbs A and C and the distance from 

 the knife edge G to the screw F. An error of o.i inch in either of 

 these measurements is an error of i per cent in head or 0.5 per cent in 

 velocity. There is no difficulty in getting these distances to the 

 nearest hundredth of an inch. The screw thread was cut so precisely 

 that it was impossible to detect any error in the pitch of the thread. 

 The hole in Z was tapped with a standard Brown and Sharp tap. 

 Ihe calculation of the change in level of the surfaces of the liquid in 



