VOL. LXXII.] PHILOSOPHICAL TRANSACTIONS. 283 



when dry, into pieces of convenient lengths. It was hoped, that this method 

 would of itself have been sufficient, without the addition of the paring gage, 

 making proper allowance, in the size of the holes, for the shrinking of the clay 

 in drying. But it was found, that a variety of little accidents might happen to 

 alter the shape and dimensions of the pieces, in a sensible degree, while in their 

 soft state ; so that it will be always safest to have recourse to the paring gage, 

 for ascertaining and adjusting their breadth when perfectly dry, this being the 

 period at which the pieces are exactly alike with regard to their future diminish- 

 ing ; so that if they are now reduced to the same breadth, we may be sure that 

 they will suffer equal contractions from equal degrees of heat afterwards, 

 whether they have been made in a mould, or by a press, or in any other way ; 

 neither is any variation in the length or thickness of these pieces of the least 

 consequence, provided one of the dimensions, that by which they are after- 

 wards to be measured, is made accurate to the gage. 



5. It will be proper to bake the pieces, when dry, with a low red heat, in 

 order to give them some firmness or hardness, that they may, if necessary, be 

 able to bear package and carriage ; but more especially to prepare them for being 

 put into an immediate heat, along with the matters they are to serve as measures 

 to, without bursting or flying, as unburnt clay would do. We need not be 

 solicitous about the precise degree of heat employed in this baking, provided only 

 that it does not exceed the lowest degree which we shall want to measure in prac- 

 tice ; for a piece that has suffered any inferior degrees of heat, answers as well 

 for measuring higher ones as a piece which has never been exposed to fire at all. 

 In this part of the preparation of the pieces, it may be proper to inform the 

 operator of a circumstance, which, though otherwise immaterial, might at first 

 disconcert him : if the heat be not in all of them exactly equal, he will probably 

 find, that while some have begun to shrink, others are rather enlarged in their 

 bulk ; for they all swell a little just on the approach of redness. As this is the 

 period of the most rapid produce of air, the extension may perhaps be owing to 

 the air having at this moment become elastic to such a degree, as to force the 

 particles of the clay a little asunder before it obtains its own enlargement. 



6. Each division of the scale, though so large as a 10th of an inch, answers 

 to -g-i-^ part of the breadth of the little piece of clay. We might go to much 

 greater nicety, either by making the divisions smaller, or the scale longer ; but 

 it is not apprehended that any thing of this kind will be found necessary : and 

 indeed, in proceeding much further in either way, we may possibly meet with 

 inconveniences sufficient to counter- balance the apparent additional accuracy of 

 measurement. 



7. The divisions of this scale, like those of the common thermometers, are 



o o 1 



