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362 Mr. Wepewoon's Method of conneéting ae 
either, but the excefs of the expanfion of one above that of the 
other, which is fufficient for the prefent purpofe, as we want 
only an uniform and graduated effect of fire, and. it is totally 
‘immaterial whether that effect be the abfolute expanfion of one 
or the other body, or the difference of the two, provided only: 
that its quantity be fufficient to admit of nice meafurement.. > 
Some difficulties occurred with refpe& to. the choice of a 
proper matter for the gage; the eflential requifites of. which 
are, to have but little expanfibility, and to bear the neceflary 
fires without injury. All the metals, except gold and filver, 
would calcine-in the fire: thofe two are: indeed free from that 
objection, and accordingly it is of the moft expanfible of them 
that the piece is made; but if the gage alfo was made of the: 
fame, the meafure itfelf would expand juft as much as the body: 
to be meafured, and no expanfion at all would be fenfible; and 
though the gage was made of one of thofe metals, and the 
piece of the other, the difference between thetr expanfions 
would be too {mall to give any fatisfactory refults, as more 
than. two-thirds ef the real expanfion of either would be 
loft or taken off by the other. 
For thefe reafons I, had secourfe to earthy comrpoGicielle 
which expand by heat much lefs than metallic bodies, and bear’ 
the neceflary degrees of fire without the leaft injury. I made: 
choice of tobacco-pipe clay, mixed with charcoal in fine pow-' 
- 
der,, in the proportion of three parts/of the charcoal to five of 
the clay by weight. By a free accefs of air, in the burning by 
which the gage 1s prepared for ufe, the charcoalis confumed, | 
and leaves the clay extremely light and porous; from which» 
circumftance it bears fudden alternations of cold and heat, often’ 
requifite in thefe operations, much better than the clay alone. 
Another and more important motive for the ufe of charcoak! 
\ Was, 
