556 Proceedings of the " 



position, the air will pass to the upper part of the cistern, and there 

 its elastic pressure on the surface of the mercury being- the same as 

 that of the atmosphere in the syphon tube, it will cause the mercury 

 to descend in that tube; and the length of the mercurial column, 

 equal in weight to the atmospheric pressure, being thus completed, 

 the mercury will descend in the upper tube, and rise in the shorter 

 leg; of the syphon ; if the instrument be inverted, the mercury will 

 return into the cistern, into which a stopper is screwed when it is 

 not in use. Each portion of the instrument is enclosed in a brass 

 tube, with a scale and vernier to read each end of the mercurial 

 column. 



A portable transit instrument, also made by Robinson, was placed 

 upon the table by Captain Grover. 



March 18th. Mr. Ritchie delivered a lecture this evening; on 

 elasticity in general, particularly the elasticity of torsion in threads 

 of glass, with the application of this property to delicate physical 

 researches. If a portion of air be compressed into a smaller bulk, it 

 endeavours to regain its former state with a force directly propor- 

 tional to the force which has been employed to compress it, or 

 inversely proportional to the bulk into which it has been compressed. 

 This power, resulting in all probability from the repulsive energy 

 of the molecules of heat with which it is combined, is termed its elas- 

 ticity, the only kind of elastic force possessed by aeriform substances. 

 If a solid body be compressed into a smaller bulk, it also endeavours 

 to regain its former state with a force proportional to that with which 

 it was compressed, provided the molecules of the body have not 

 undergone a permanent displacement. This is called the elasticity 

 of compression. If a rod or wire be stretched, it will, within certain 

 limits, also endeavour to regain its former length with a force equal 

 to that with which it has been stretched, and directly proportional to 

 the increments of length which it has received. This is called its 

 elasticity of tension. When a rod is bent, the atoms on one side 

 have suffered compression, and on the other side extension ; hence 

 both these forces will act in restoring the rod to its former state, with 

 a force proportional to the degree of flexure which it has undergone. 

 If one end of a wire of iron, brass, steel, &c. be fixed, and the other 

 twisted round, the wire will endeavour to return to its former state 

 with a force directly proportional to the number of degrees through 

 which it has been twisted, provided the atoms have not suffered a 

 permanent displacement; or, in other words, provided the wire has 

 not taken a set. Coulomb was the first person who investigated 

 the nature of torsion belonging to metallic wires, and employed 

 this property in a beautiful manner in his torsion balance. The 

 celebrated Cavendish also employed it to determine the attraction 

 of leaden balls, and thence the attraction, and consequently the 

 specific gravity, of the earth itself; so that, to use the words of a 



