VOL. LXXVI.] VHILOSOPHICAL TRANSACTIONS. 145 



other period ; but proceeds in one uninterrupted course of diminution, from the 

 soft state in which the pieces are formed, up to the extreme fires of our fur- 

 naces. Though the diminution however is uninterrupted, it is at the same time 

 so inconsiderable at the beginning, from the heat of boiling water, at which the 

 pieces are adjusted, up to ignition, that the same point of visible redness is taken 

 for the commencement of the scale, in this as in the original clay, without any 

 sensible error or variation in their progress, 



I am inclined to believe, though experiments have not yet enabled me to speak 

 with certainty on this point, that the same cause which enlarges the natural 

 clays on their first exposure to the fire, operates also in this composition, but in 

 a much lower degree ; that while the natural clays have their whole mass dis- 

 tended by the efforts of the air in forcing its passage, the composition is only 

 restrained in its diminution, or prevented from diminishing so fast as it other- 

 wise would do, and as it is found to do in the subsequent part of its course, 

 after the air has escaped from it. 



As the composition of clay and alum earth is far more tenacious of water than 

 the clay itself, and was found, after being dried by the heat of boiling water, to 

 yield, by distillation in a retort, above 3 times as much aqueous fluid as the 

 original thermometric clay did ; it seems probable, that a part of this water, re- 

 tained to the approach of ignition, and in a state of chemical combination, may 

 facilitate the passage of the air, serving as a vehicle to convey it off through in- 

 terstices not permeable to air alone, and consequently enabling it to escape 

 without doing that violence to the mass, which the natural clays sustain from the 

 expulsion of their air after the water has been detached from it ; for the experi- 

 ments of Dr. Priestley have shown, that vessels even of burnt clay are permeable 

 to air when they have imbibed water into their substance, though not at all so in 

 a dry state. 



XXIII, The Latitude and Longitude of York determined from a Variety of 

 Astronomical Observations ; together with a Recommendation of the Method of 

 determining the Longitude of Places by Observations of the Moons Transit 

 over the Meridian, By Edward Pigott, Esq. p. 40Q. 



The difference of meridians between Greenwich and York was found by the 

 following methods : viz. 1st. by occultations of stars by the moon ; 2dly, by ob- 

 served meridian right-ascensions of the moon's limb ; 3dly, by observations of 

 Jupiter's 1 st satellite ; 4thly, by a lunar eclipse. By the 1st method, viz. oc- 

 cultations of stars, the difference of meridians between York and Greenwich was, 

 by a medium of the observations, 4"^ 27^ ; in like manner the medium by the 

 right ascensions gave 4™ 24*4-; that by Jupiter's satellites 4" 31'; and that by 



VOL. XVI. U 



