20 OF THE PROPERTIES OF MATTER. 



LECT woollen cloth till it feels warm ; and then, laying 

 ^x-v-x^ it on where it was, observe how far it pushes the point 

 S upon the scale by means of the crooked index El; 

 and the point S will shew exactly how much the bar 

 has lengthened by the heat of rubbing. As the bar 

 cools, the spring R bearing against the index K G, 

 will cause its point S to move gradually back towards 

 M in the scale : and when the bar is quite cold, the in- 

 dex will rest at M, where it was before the bar was 

 made warm by rubbing The indexes have small rollers 

 under them at 7 and K; which, by turning round on 

 the smooth wood as the indexes move, make their mo- 

 tions the easier, by taking off a great part of the fric- 

 tion, which would otherwise be on the pins F and H, 

 and of the points of the indexes themselves on the 

 wood. 14 



Magnetism Besides the universal properties above mentioned, 

 there are bodies which have properties peculiar to them- 

 selves : such as the loadstone, in which the most re- 

 markable are these : 1. It attracts iron and steel only 1S 



Note 14. The pyrometer here described, has been justly considered 

 as defective both in principle and execution. If it be merely intend 

 ed to illustrate the expansion of metals by heat, the fact mav be 

 shewn by fitting a piece of large wire into a hole, and it will be found 

 that after it has been brought to a red heat, that its bulk will have 

 so far increased, as to prevent its passing through an aperture. A 

 reference to this circumstance will serve to account for the change of 

 form which takes place in bridges and other edifices constructed of 

 iron. When it is found necessary however, to ascertain the compara- 

 tive amount of expansion in various metals under the same degree of 

 temperature, a variety of more accurate instruments may be resorted 

 to : one of these will be described in a future page. 



Note 15. For iron and steel, we may here substitute ferruginous bo- 

 dies generally, as subsequent experiments have shewn that there is 

 scarcely any body that is not operated on by the magnet To discover 

 the amount of attraction in substances but very slightly magnetic, it i 

 necessary to employ a paper or cork float in a vessel of water, and upon 

 this the body under examination should be placed so as to prevent the 

 water wetting its upper surface, and the relative distance, at which this 

 is acted upon, in proportion to its hulk, determines the magnetic power. 



