598 Mr. cavallo’s 'Thermometrical 
which was the 40th degree, coincided with the line, 
which went from the point g to the 40th degree on the 
fcale, and the mark k coincided with the line which went 
from g to the 70th degree. In that place the thermome- 
ter was left, and in order to keep it fteady, a piece of lead 
(o) was ufually put upon the extremity of its tube. In 
this fituation the interfedtions of the tube, and the lines 
drawn from G to the degrees in ef, fhewed the degrees, 
or ferved for a fcale to the thermometer hl. Thus, fup- 
pofe that the quickfilver in its tube was at p, it is plain, 
that it was at 55 0 , becaufe at that point the tube is inter- 
fered by the line which goes from g to the 5 5th degree 
on the fcale ef. This board, therefore, ferved for a uni- 
verfal fcale, and upon it I ufed to fix feveral thermome- 
ters at a time,, and expofe them all together to the Sun. 
I Ihall conclude this paper with mentioning an experi- 
ment, which, although not thermometrical, is yet ufeful 
in removing a wrong notion fome perfons have con- 
cerning the effedt of light. 
Having feen in fome book that the common black 
pyrophorus, or home, erg’s pyrophorus, was impaired 
by light, I was defirous to try the truth of this aflertion. 
Accordingly towards the beginning of the laft year I 
prepared fome pyrophorus, and inclofed portions of it 
in three glafs tubes, which were immediately fealed 
1 hermetically, 
