22 OF CENTRAL FORCES. 



tance ; approaching to the one or the other, as the mag- 

 nitudes of the attracting bodies are varied. 

 Electricity Several bodies, particularly amber, glass, jet, sealing- 

 wax, agate, and almost all precious stones, have a pecu- 

 liar property of attracting and repelling light bodies 

 when heated by rubbing. This is called electrical at- 

 traction, in which the chief things to be observed are, 

 1. If a glass tube about an inch and a half diameter, 

 and two or three feet long, be heated by rubbing, it will 

 alternately attract and repel all light bodies when held 

 near them. 2. It does not attract by being heated with- 

 out rubbing. 3. Any light body being once repelled by 

 the tube, will never be attracted again till it has touched 

 some other body. 4. If the tube be rubbed by a moist 

 hand, or any thing that is wet, it totally destroys the 

 electricity. 5. Any body, except air, being interposed, 

 stops the electricity. 6. The tube attracts stronger 

 when rubbed over with bees-wax, and then with a dry 

 woollen-cloth. 7. When it is well rubbed, if a finger be 

 brought near it, at about the distance of half an inch, 

 the effluvia will snap against the finger, and make a lit- 

 tle crackling noise ; and if this be performed in a dark 

 place, there will appear a little flash of light." 



As the expansion of metals by heat was but briefly examined 

 a.tpage 18, it has been deemed advisable to reserve the follow- 

 ing valuable table for the end of the Lecture. It shews, in 

 parts of an inch, how much one foot length of different substan- 

 ces is expanded by 180 of heat, (Fahrenheit's scale,) between the 

 freezing and the boiling points of water. To the first seven substan- 

 ces, (which were examined in Mr. Ramsden's most accurate 

 pyrometer,) there are added the expansions for a single degree 

 of heat. The others were determined by Mr. Smeaton with his 

 pyrometer. 



Note 10. See Ferguson's Introduction to Electricity, with notes, &c, 

 now preparing tor publication. 



