Strehlke on the Acoustic Figures of Plates. 293 



tween the two figures was very different, so much so that 

 whilst, on the one surface, the curves evidently came only near 

 each other, it was doubtful whether the lines on the other 

 surface were intersected or not. Great attention is further to 

 be paid to other circumstances which might influence the ex- 

 periment, for the least change of temperature, a very slight 

 degree of humidity, the commencement of oxidation, or any 

 external agitation in the vicinity of the plate, is sufficient to 

 disturb the formation of distinct figures. The quantity of 

 sand * is also of great importance in that respect, and ought 

 not to exceed above three or four grains on a square line ; the 

 bow must be moved up and down steadily, and until the figure 

 has ceased to undergo any further change ; and if the figures 

 are to be measured, it is indispensable to reproduce them as 

 long as they are not formed by one row of grains only, the 

 joint central line through which may then be regarded as the 

 quiescent line ; for if there were several grains, three, for in- 

 stance, we should by no means be justified in considering the 

 middle one as the representative of the quiescent line, as either 

 of the two others might in reality stand for it ; for it is as pro- 

 bable that one of the outer grains is balanced by the two 

 others, as that the middle one is fixed to the two outer grains, 

 &c., for that the plate rests in mathematical lines is, we believe, 

 universally admitted ; and although sometimes, if much sand 

 be used, broad lines are formed, the outer rows may always be 

 seen to move as long as the plate sounds, whilst one line is 

 completely quiescent. 



In* the following experiments the plates were supported on 

 one side only, either on a vertical wooden bar with a small 

 piece of cloth at the point of contact, or merely on the spread 

 fingers of the left hand ; either of these two methods is pre- 

 ferable to the use of the screw, which scarcely admits of the 

 reproduction of the same figure ; for the least difference in the 

 tension or in the plane where the plate is fixed, changes the 

 figure, &c., which is not the case if the plate is supported in 

 the manner above described, where it is sufficient that the 



* Professor Strehlke always used sea-sand, the grains of which, under the micro- 

 scope, appeared as spheroids from 0"'.U3 to 0" / .U5 diameter, 



