t 412 3 
lions were to be the removal or approach of its fibres 
to each other. 
20. Being thus guided by thefe neceffary condi- 
tions, I thought of different thin ivorv cups, the ca- 
pacities of which fhould be meafured by quickfilver ^ 
and at laft imagined a hollow cylinder, in which the 
variation of its capacities, when more or lefs moiff, 
might be meafured by the quickfilver it fhould be 
filled with; and which putting into a glafs tube 
joined to the ivory one, would of coui fe rife more or 
lefs, as that veffel was more or lefs deprived of humor. 
21. Nothing now remained but to find out a way 
of eftimating the changes of capacity of the ivory 
tube, by means of the variations in the height of the 
mercury in the glafs tube. 1 thought, at firft, that by 
ufing very nice feales, in order to compare the 
weights of the mercury contained in the cylindrical 
veffel, with that of a column of the fame liquid in 
the tube, I might obtain the proportions of thefe 
weights with a fuflicient exaftnefs, to be able to mea- 
sure the variations of the mercurial column, by de- 
grees reprefenting aliquote parts of the whole mafs. 
22. This in itfelf was undoubtedly an exadf me- 
thod ; but then it required in the execution fuch a 
nicety in the feales, .that 1 durft not employ it in the 
conftru&ion of an inftrument of fo extenlive an ufe.. 
Such feales are always fcarce from their high price. 
I remembered to have myfelf found that inconve- 
nience in the conftru&ion of a Delifle’s thermo- 
meter, and concluded I muff hit uponfome method, 
to. avoid it. 
23. The idea of a thermometer, which ffruck 
my mind, was a. lucky one* I was led to it by a 
kind; 
