Fig. 4. 



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542 Mr. Gr. A. Shakespear o?i an Interference- Method for 



plate with conical ends. At the upper apex a steel hook 

 fitted into a screw-stopper of brass; from the lower apex 

 hung a stout brass wire which bore four vanes (fig. 4). 

 The whole hung freely in a cylindrical vessel of tin-plate, 

 and down one side of this vessel passed a tube which 

 opened downwards in the middle of 

 the bottom. When the outer vessel 

 is filled with water up to the level of 

 the suspending hook the weight hanging 

 on the wire depends on the amount of 

 shot placed in the cylinder (by way of 

 adjustable weight). By siphoning out 

 through the tube the water from the 

 outer vessel until the level falls to the 

 lower wire (lower dotted line in fig. 4), 

 a weight equal to that of the water dis- 

 placed by the cylinder is applied gra- 

 dually without vibrations. The object 

 of the conical ends is to ensure that the 

 application or removal of the force is 

 begun gradually and without jerk, so 

 that the eye has no difficulty in observing 

 the movement of the fringes from the outset. (The sudden 

 making or breaking of the surface of the water over a flat 

 end sets up perilous vibrations. A similar method which 

 was abandoned in favour of this was the use of a true cylinder 

 suspended in water contained in another concentric cylinder, 

 and the weight applied was got by measuring the fall of water 

 in the outer vessel.) 



The vanes are to damp vibrations when the water is at the 

 lower level. An indiarubber tube from the outer vessel led 

 to a tap, whereby the flow of water could be regulated, and 

 from the tap another length of tube passed to a bucket into 

 which the water flowed ; by raising this bucket the water 

 could be siphoned back again and the contraction of the wire 

 observed. 



Vibrations. — These gave much trouble at first, and necessi- 

 tated night work. With a view to eliminating this trouble, the 

 work was done in a cellar at the Cavendish Laboratory, and 

 from the floor two piers of masonry were built up ; on these 

 stood a massive stand of cast iron (weighing about 3 cwt.) on 

 indiarubber blocks. On the top of this stand (part of 

 which can be seen in fig;. 5) a thick disk of cast iron was 

 supported on three levelling-bolts which rested in three V 

 grooves. From the centre of this disk a one-inch steel rod 

 passed down through the top of the stand and supported the 



