of Ekclrkitij bij Piessurc. 211 



to be established between the two bodies, then, on the pressure 

 ceasing, neither of them will have acquired any electricity. 

 It is therefore clear that the development of electricity in this 

 case takes place only during the passage of caloric from the one 

 body to the other; as soon as that has ceased, we see no more 

 electric effects. Thus, then, when two bodies pressed one 

 against the other retain no sensible electricity alter compres- 

 sion, before we pronounce on their want of the electric property 

 we must ascertain v/hether a change of tempeiature in one of 

 them would not suffice to render them electric. 



The hygrometric water which usually adheres to the surface 

 of bodies, sometimes destroj's the electric property of pressure: 

 for example, suljjhate of barytes, sulphate of lime, mica, &,c. 

 must be freed from this water before they are subjected to the 

 experiment; without this precautiouj no development of 

 electricity will be obtained: for want of having taken it, some 

 philosophers have concluded that these substances were not 

 electric by pressure. In certain cases it is necessary to attend 

 to the dimensions of the disks: for instance, when Iceland spar 

 is pressed with an insulated disk of metal, if this disk be of a 

 certiiin size, the development is null ; while if it is a milli- 

 metre in diameter, the spar immediately acquires an excess of 

 vitreous electricity. Want of polish in Iceland spar entirely 

 changes its electric properties ; from being a very bad con- 

 ductor it becomes a good one; so that it is necessary to insu- 

 late it, in order to make it preserve the electricity it has acquired 

 by pressure ; its electric sensibility is then considerable. 



To recapitulate: — We find that the electric effects of pres- 

 sure are modified by the temperature of bodies, by the i-a- 

 pidity with which they are sepai-ated, by their hygrometrical 

 state, by the state of the particles of their siu-faces, &c. 

 [To be continued.] 



I 



XLIII. On the Nature of the Curves described by one of the 

 Combinations of Jopling's Apparatus for describing Curves. 

 By Mr. Thomas Tueugold. 



To the Editors of the Philosop)hical Magazine andjomiial. 

 N your last Number a notice was given of a very simple and 

 general method of describing curves, invented by Mr. Jop- 

 ling. Of this method 1 projiose to take a single case to con- 

 sider, and one of the easiest; leaving the odiers to those who 

 arc better acquainted with tlie doctrine of curves, and better 

 versed in the art of analysis. 



The case I propose to investigate may be thus stated : Sup- 

 pose there to be two straight lines on a fixed plane, and two 



D d 2 fixctl 



