VOL. LXXVI.] PHILOSOPHICAL TRANSACTIONS. 13/ 



true cause of the inequality, I was convinced by firing some pieces unadjusted, 

 with all their surfaces entire, as they came from the mould ; for these pieces, 

 after passing through the same strong fires with the preceding, continued flat, 

 with the angles regularly sharp, and without the least sensible prominence in 

 any part. 



Some of the moulds, employed for this use, were made of plaster, a material 

 more convenient for the workman than metal, as the pieces part more freely 

 from it, but which contributed greatly to increase the above-mentioned irregu- 

 larity : for the plaster, by absorbing a portion of the water from the clay conti- 

 guous to it, renders the surface at the same time, even at the instant of contact, 

 much more consistent, and consequently more difficult to press into the angles 

 of the mould ; so that the outsides of these pieces were not only more com- 

 pressed, but formed of clay of a different temper from the inner parts, being 

 much drier or firmer, a circumstance which restrains still more their diminution 

 in the fire. 



The moulds were therefore laid aside, and the press adopted in their stead ; 

 for as the soft clay, pressed in a cylindrical vessel, gives way and escapes through 

 an aperture made for that purpose (by which means it is formed into long rods), 

 the sides of the piece cannot be supposed to receive so great a degree of corn- 

 pressure against the sides of the aperture through which it is delivered in this 

 operation, as it does against the sides of the mould, by which it is confined till 

 every part has born a pressure sufficient to force the clay into every angle, which 

 is much greater than even a workman would imagine till he comes to try the 

 experiment himself. But with this change some new difficulties arose ; for 

 pieces pressed through the same aperture, and from the same press-ful of clay, 

 and adjusted, when dry, to the same point in the gage, were found, after passing 

 together through the same strong fires, to differ in their dimensions from each 

 other, in some instances more than any of the preceding. 



Having hitherto paid no particular attention myself to the mere manual labour 

 of pressing the clay, I determined on this event, to go through that and every 

 other operation, however simple and seemingly insignificant, with my own hands. 

 In doing this I observed, that the power necessary for forcing the clay through 

 an aperture which bore but a small proportion to the diameter of the mass of clay 

 in the press, was so great as to squeeze out, along with the clay that first passed 

 through, a considerable portion of the water that belonged to the rest. From 

 this over proportion of water in the composition of the first pieces they were soft 

 and spongy, and the succeeding ones more and more compact, till at length the 

 clay proved so stiff as scarcely to be forced through at all. 



Clay, containing different proportions of water, is well known to diminish 

 differently in drying ; but it was not imagined that, when dry, there would beany 



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