406 PHILOSOPHICAL TRANSACTIONS. [aNNO 1747. 



just before its bottom runs ; and he imagines, that an instrument may be con- 

 structed with supporters, and a rod made of tobacco-pipe clay,* which, being 

 regulated by boiling mercury, for it must never touch water, may be adapted 

 to measure still greater degrees of heat, till the materials themselves melt into 

 glass. 



Dr. M. advises, that not only the scale of this kind of thermometer, but also 

 of all others, be determined by experiments, without regarding any equality as 

 to measure between the divisions, and that in every individual that shall be made; 

 for a difference in the length and thickness of the rods will make a difference in 

 the scale, as much or more than the inequality in the cavity of the stem, or glass 

 tube of other thermometers, which can never be just, if applied to a scale wliose 

 divisions are made equal ; unless the cavity of the stem be perfectly equal, which 

 it is impossible for any workman to do, and which is very seldom, if ever, hit on 

 by chance. Therefore, in these instruments, let the point b in fig. 3, or the 

 horizontal position of the index, be the situation of the index when the rod has 

 stood a quarter of an hour in boiling water; there mark \7 boiling on the outer 

 circle; on Fahrenheit's arch mark 212. Then set the machine up to the mark 

 + into melting tin, which is the metal that melts easiest. When the rod is ar- 

 rived to its greatest elongation in that metal, inscribe the character Tf, on the 

 outer circle; do the like with lead, and set the character h, at it. At the boiling 

 of mercury put the mark 5 » and on Fahrenheit's arch mark 600, the utmost 

 extent his mercurial instruments can measure: then proceed to the melting of 

 silver, and set the mark ([ ; at the melting of gold place the mark ; at the 

 melting of copper place the mark 9 ; at the melting of iron place the mark fj , 

 the most difficult to melt of all metals. 



As the divisions pointed out by the index will be different with rods of dif- 

 ferent metals or substances, we may make different circles on the plate for the 

 range of the different rods, and mark them ; the iron rod, the brass rod, the clay 

 rod; and set the several marks above specified on each circle apart; or, to avoid 

 confusion, we may have a different instrument for each kind of rod. 



According to Fahrenheit's scale, the heat of the strongest sunshine is at about 

 80; spirit of wine boils at 176; water at 212; the lixivium of salt of tartar at 

 240; spirit of nitre at 242; oil of vitriol at 546; quicksilver at 60O.-|- 



As all chemical digestions, where an equable heat is to be continued for some 

 time together, will come in between hot sunshine and the boiling of quicksilver, 



• This idea, of using argillaceous bodies as thermometers, for measuring very high degrees of 

 heat, was afterwards further acted on by Mr. Josrali Wedgwood, in vol. 72, &c. of tlie Philosophical 

 Transactions. 



+ See Augustin. Grischow Thermometria comparata accuratius, et Harmonica. Beroiini 1740, 4to. 

 .p. 10.— Orig. 



