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Fi^. 5o is a .■-ketch of a curious form of double-freedom seismometer, in 

 wliieli a somcwiiat similar method of suspension is used. The base S, which is 

 fixed, is au o(iuilateral triangle, at the corners of which there are three conical 

 cups forming sockets for three legs rrr of equal length. These legs press up 

 into three other sockets in tlie plate s, which are placed so as to form an equi- 

 lateral triangle of half the linear dimensions of the base. The height of s from 

 tJK' base is 0.8(JG times the distance between the base sockets. The plate s carries 

 a massive bob if, slightly lieneath it. The plate is then in nearly neutral (.some- 

 what stable) equilibrium with respect to small motions in any azimuth. It is 

 neces.sary iiere to invert the S3'stem and use struts instead of ties, since a plate 

 hung by three cro.ssed cords would be azimuthally unstable. Two of the legs 

 are made with loops to allow the three to cross each other. By adding a multi- 

 ])lying ))ointer to record the displacements of the base with respect to the centre 

 of the |)]ate, we should obtain a compact form of double-freedom seismograph, 

 whose fVictional resistance (though mueli greater than that of the single-freedom 

 instrument just described) could probably be kept within reasonable limits. 



