Lateral Vibration of Bars. 585 
up immediately in front of a white screen ruled in squares. 
The bar is set in motion by drawing aside and then releasing. 
At first the bar was pulled by the end, but afterwards it was 
always pulled by a point one-fifth of the way down. After 
a good number of swings the bar is photographed when near 
the full extent of its swing. From the negative we know 
the position of the bar when taken. J aAtas then pulled aside 
by a force applied at any point and held in such a position 
that the end is in the same position as when photographed in 
motion. A photograph is then taken of the bar held thus, 
the camera being in exactly the same position as before. 
We thus obtain one photograph of the dynamical curve (as 
we may call the shape of the vibrating bar) and a number of 
statical curves. By double printing we compare the former 
with each of the latter. : 
The first photograph (Pl. XVII. fig. 2) shows the bar vibra- 
ting, compared with the same bar held aside bya force at the 
end. The conditions of the experiment make the two curves 
coincide at the ends, but near the middle they are quite 
distinctly separate. The vibrating rod appears on the 
outside. 
In the second photograph (fig. 3) the bar is held by a 
force applied one-tenth of the way along the bar from the 
free end. The two curves almost coinvide, a slight divergence 
being indicated by a thickening of the apparently single 
line. . 
In the third (fig. 4) the force is applied one-fifth of the 
way down. Here we have almost perfect coincidence. 
In the fourth (fig. 5) the force is applied three-tenths of 
the way down. We have again two distinct curves, as in 
the first case, but now the vibrating bar appears on the 
inside. 
INisplacement observations—A second method of expe- 
riment was adopted, which we may call the microscope 
method. 
It will be seen from the photographs (as might be expected) 
that the greatest divergence between the two curves is near 
the middle of the bar. Then we can make comparisons 
between the curves by direct observations of the displacement 
of the bar from its mean position (1) at the free end, and (2) 
at the middle. This was done as follows :—two traversing 
microscopes were focussed on the bar when pulled aside, one 
at the free end and the other at the middle. It was found 
that if we start the bar vibrating by pulling it aside to a fixed 
block and releasing, the amplitude after a given number of 
swings will be a fixed amount, and the condition of things 
