On a Modification of Wkeatstone's Rheostat. 29 



It is submitted that, considering the nature of the subject, 

 these results are fairly confirmatory of the theory, and also of 

 the assumed values of the densities, and to a less extent of 

 that of the mean thickness of the crust at the sea-coast. 



Geological changes of level are closely connected with the 

 theory of hydrostatic equilibrium, as I have pointed out in 

 my i Physics of the Earth's Crust,' chap. xvii. When the 

 effects of contour have been allowed for, we ought to find a 

 general negative variation of gravity in such regions as are 

 now disposed to rise, because that would indicate that the root 

 is too deep for equilibrium. And for the s opposite reason, 

 a positive variable ought to be met with where depression is 

 in progress. Whether this relation holds has, I suppose, not 

 been noticed. Again, if the hypothesis is true, beneath plains 

 the increment of underground temperature ought to be more 

 rapid where the variation of gravity is positive, and more 

 slow where it is negative. 



Postscript. — Since the foregoing was sent to press, I have 

 read M. Faye ? s article, " Sur la Constitution de la Croute 

 Terrestre/' in Comptes Rendus for March 22, 1886. He 

 there discusses pendulum-observations at island and conti- 

 nental stations, the latter with especial reference to the Indian 

 observations. Respecting the excess of gravity found at 

 island stations, he comes to a conclusion similar to that 

 arrived at by Pratt in reference to Minicoy (art. 74). With 

 regard to continents, his views likewise agree generally with 

 those advanced by Pratt ; see in particular Pratt's art. 192. 

 It would occupy too much space to examine here the cause to 

 which M. Faye attributes the greater density of the suboceanic 

 crust.— 0. F. 



II. On a Modification of Wheatstone's Rheostat. 

 By Shelford Bidwell, M. A* 



IT is frequently desirable that the resistance of a circuit 

 through which a current of electricity is flowing should 

 be made to vary continuously and not by steps, as is necessarily 

 the case when resistance-coils are used alone. An instrument 

 for effecting this object was devised by Sir Charles Wheatstone 

 in 1843, and was called by him the Rheostat. Two forms of 

 the apparatus, both of which are well known, are described in 

 his paper on the " Constants of the Voltaic Circuit," originally 

 published in the Phil. Trans, and contained in the Physical 

 Society's Reprint of Wheatstone's Scientific Memoirs. 

 * Communicated by the Physical Society : read May 8, 188G. 



