Joty— Useful Methods of Teaching Elementary Physics. 221 
The submerged tube must be pressed up still by this force, 7.e. by 
a force equal to the weight of water displaced, Q.H.D. 
Observations upon floats of various cross-sections furnish useful 
and well-recognized exercises for the student. 
NOTE ADDED IN THE PRESS. 
In measuring volumes in the form of Boyle’s tube described 
above, it conduces to accuracy to leave a little mercury standing in 
the lower end of the tube, and thus avoid the necessity of making 
assumptions as to the volume of the hemispherical extremity of 
_thetube. The tube is thus virtually converted into a flat-ended 
tube. 
