in Free Surfaces of Liquids. 



135 



Fig. 1 



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3. Hence, after a long series of preliminary experiments, I 

 employed the following method, which, on the whole, satisfies 

 the requirements above stated. Suspended by two fine, well- 

 annealed platinum wires, a brass cross hangs bifilar, carrying 

 a small mirror for the observation of the oscillations with scale 

 and telescope. The two horizontal arms of the cross are fur- 

 nished with screw-threads, on which suitable weights can be 

 shifted, so that the oscillation-period can be varied at pleasure. 



To the descending part suitable bodies can be fastened, 

 which are destined to execute oscillations within the liquid 

 (see fig. 1). I have used for this purpose chiefly thin plates 

 or cylinders of brass. The 

 entire apparatus is suspended 

 on a suitable frame, and with 

 the aid of a micrometer- 

 screw could be raised and 

 lowered at pleasure without 

 being at the same time ro- 

 tated about its vertical axis. 

 On the micrometer- screw a 

 rise amounting to only 0*01 

 millim. could be read off. 

 Lastly, a small magnetic 

 needle was attached to the 

 apparatus, with the aid of 

 which the above-described 

 mechanism could be set in 

 steady rotatory oscillation. 

 The oscillation-period and 

 the diminution of the oscilla- 

 tions can then, in the well- 

 known manner, be determined with greater precision. The 

 experiments were executed as follows : — The plate was at first 

 entirely immersed in the liquid, and its upper margin was at 

 a determined distance from the free surface ; it was afterwards 

 raised till its upper margin was in the free surface or projected 

 a determined distance above it. 



In both cases the liquid opposed a certain resistance to the 

 plate, which produced a diminution of the amplitudes. If the 

 friction of the parts of the liquid in the free surface is greater 

 than in the interior, this resistance will also be greater. But 

 in order to observe it in the most favourable form, it was 

 necessary to give to the vessel destined to hold the liquid the 

 most suitable shape possible. It was rectangular, 150 millims. 

 in length, and 30 millims. wide. The long sides consisted of 

 glass plates. Besides, it was so contrived that two other glass 



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