VOL. LXXII.] PHILOSOPHICAL TRANSACTIONS. 281 



proposed : the clay is first to be washed over, and while in a dilute state passed 

 through a fine lawn. Let it then be made dry, and put up in boxes.* 



The dry clay is to be softened, for use, with about ■§. of its weight of water ; 

 and formed into small pieces, in little moulds of metal, -^-of an inch in breadth, 

 with the sides pretty exactly parallel, this being the dimension intended to be 

 measured, about vV of an inch deep, and 1 inch long. To make the clay de- 

 liver easily, it will be necessary to oil the mould, and make it warm. These 

 pieces, when perfectly dry, are put into another iron mould or gage, consisting 

 only of a bottom, with two sides, half an inch deep ; to the dimensions of 

 which sides the breadth of the pieces is to be pared down. 



For measuring the diminution which they are to suffer from the action of fire, 

 another gage is made, of two pieces of brass, 24 inches long, with the sides 

 exactly straight, divided into inches and tenths, fixed half an inch asunder at 

 one end, and -£>- at the other, on a brass plate ; so that one of the thermometric 

 pieces, when pared down in the iron gage, will just fit to the wider end. Let us 

 suppose this piece to have diminished in the fire J- of its bulk, it will then pass 

 on to half the length of the gage ; if diminished -f- it will go on to the narrow- 

 end ; and in any intermediate degree of contraction, if the piece be slid along 

 till it rests against the converging sides, the degree at which it stops will be the 

 measure of its contraction, and consequently of the degree of heat it has 

 undergone. 



These are the outlines of what appears to me necessary for the making and 

 using of this thermometer; and it is hoped, that the whole process will be found 

 sufficiently simple, and easy of execution. It may however be proper to take 

 notice of a few minuter circumstances, and to mention some observations which 

 occurred in the progress of the inquiry. 1 . There ought to be a certainty of 

 the clay being easily, and at all times, procurable in sufficient quantity, and on 

 ' moderate terms. That this is the case with the clay here made choice of, will 

 be evident to every one acquainted with the natural history of Cornwall, where 

 there are beds ot this clay inexhaustible, and in too many hands to be monopo- 

 lized. If this should not prove satisfactory, the author offers to this illustrious 

 Society, and will think himself honoured by their acceptance of, a sufficient 

 space in a bed of this clay to supply the world with thermometer-pieces for nu- 

 merous ages ; and he does not apprehend, that any greater inconveniences can 

 arise to foreign artists or philosophers, from their being supplied with clay for 

 these thermometers from this spot only, than what we now feel from being sup- 



* While the clay is thus kept dry in boxes, as well as while it continues in its natural bed, it is se- 

 cure from alterations in quality, which clays in general are subject to undergo, when exposed, for a 

 long course of years, to the joint actions of air and moisture. — In the lawns I made use of, the inter- 

 stices were each less than the 100,000 parts of an inch. — Orig. 



VOL. XV. O o 



