138 PHILOSOPHICAL TRANSACTIONS. [aNNO l/SS. 



difference in its subsequent diminutions by fire. Experiments however, multi- 

 plied in a variety of circumstances, showed decisively, what the pieces formed in 

 the mould had given grounds to suspect, that those formed of the softest clay, 

 and which had undergone the least pressure, diminished most in burning ; and 

 that the diminution is uniformly less and less, in proportion to the greater degree 

 of pressure or compactness. The knowledge of the cause of the irregularity 

 suggested a remedy. I lessened the width of the press very much, so as to 

 bring the diameter of the mass of clay, and that of the aperture through which 

 it is delivered, to a nearer proportion with each other. A much less degree of 

 force being now sufficient, the pieces, or rods, were proportionably more uni- 

 form, though there was still a sensible difference, in consistence, between those 

 which were first and last pressed out from the same mass of clay. The interme- 

 diate ones, within a certain distance from the two extremes, corresponded very 

 nearly with each other ; so that by rejecting a sufficient number of the first and 

 last, and using the intermediate ones only, the inequality may be considered as 

 almost annihilated. 



Yet I still found that, in strong fire, the edges became a little prominent, 

 though not so much as before. I was aware that these pieces must partake, in 

 some degree, of the imperfection of those made in the mould ; having their 

 surfaces rendered, by their friction against the sides of the aperture, more com- 

 pact than the inner paft. But I suspected that something might depend also on 

 the form, and accordingly made many trials for ascertaining the form that might 

 be least liable to this irregularity : the angles only were bevilled off, the sides 

 were rounded, the pieces were rounded all over, made of ovals and other curves, 

 and both the longest and shortest dimensions were used as the extent to be mea- 

 sured: the general result was, that the nearer they came to a circular figure, 

 the less inequality they contracted in the fire, and by making them entirely 

 circular, the imperfection appeared to be obviated altogether ; cylindric pieces 

 bearing the strongest fires without the least appearance of prominence or in- 

 equality in any part of their surface. I have therefore chosen this last form, 

 leaving only one narrow flat side (ab, fig. 8) as a bottom for the pieces to rest 

 on, and to distinguish the position in which they are to be measured in the gage. 



I have endeavoured at the same time to obviate whatever inaccuracy the 

 inequality of compactness may be capable of producing, by so adjusting the 

 aperture through which the rods are pressed, and on which their figure and 

 dimensions depend, as to supersede the use of the paring gage altogether ; that the 

 whole surface may remain of the same uniform oompactness which it received 

 in the press. And as it is scarcely practicable, in any mode of ibrming soft 

 clay, to have all the pieces precisely of the same dimensions after drying, I do 

 not reject those which come within 2 or 3 degrees of the standard, but, instead 



