21$ NOTICES CONCERNING PHILOSOPHICAL APPARATUS 



" The outfide bit taken off, but drops when the infide one 



flicks, but drops when the outfide one is put on. Apply the 



magnet as directed, the ftrokes to be north to north ftrait acrofs 



the infide, very difficult to gain, but may be otherwife varied 



at pleafure." 



has four poles When I firft had it, the middle piece would not drop off 



and fcur neutral when t j ie outer was put but on the con trary, the outer 



points, which r . , " . 



•re changed in was difengaged by application of the inner piece, and this is 



their power by the cafe at prefent. I could not obtain any explanation of the 

 obfcure writing I have copied. The fketch fhows where the 

 poles were at firft and ftill are placed, being four in number, 

 fo that it feems to be two horfeflioe magnets joined by a fhort 

 bar nearly ftrait. It barely held its own weight by the outer 

 piece, and much lefs by the inner ; but now, whether by the 

 courfe of time in which it has been carelefsly thrown in a 

 drawer among other metallic matters for fixteen years, or whe- 

 ther from any other caufe, its attraction is frrongeft at the inner 

 extremities though not fufficient to bear its weight. 



Galvanic Apparatus. 

 Davy's apparatus Plate X. Fig. 2 and 3. reprefents the apparatus of Mr. Davy 



^ffi/ 1 for takin § the g alvanic f P ark in the g al * es *• 



Fig. 1 . reprefents the apparatus for taking the fpark. A is 

 a tube graduated to grain meafures. C is a platina wire her- 

 metically fealed into the tube and having a piece of charcoal 

 attached to its top. B is a moveable platina wire, having 

 charcoal at the top ; the effect is produced by making the con- 

 tact between the piece of charcoal. In cafes where the fluids 

 are very imperfect conductors, the wires may be ufed without 

 the charcoal. 



Fig. 2. reprefents the apparatus for taking the fpark in gafes, 

 it is ufed over mercury. A and B are the communicating pla- 

 tina wires to which the charcoal is fattened, and C is the gra- 

 duated tube in which the gas is acted upon. 



* Royal Inftitution, I. 214. 



XIV. Experiment 



