298 Mr. mudge on the ConJlruBion 
whole compofition, it will then have its utmoftwhitenefs; 
but will he rendered at the fame time fo exceedingly 
hard and brittle, that the fineft wafhed emery upon lead 
or brafs wfill not cut it without breaking up its furface ; 
and the common blue ftones ufed in grinding the fpe- 
culum, w ill not touch it. Mr. jackson (fome time fince 
dead) a mathematical-inftrument-maker, and a moft ex- 
cellent workman, told me, that the tin was increafed to 
the above proportion in his metals; but that they were 
fo exceedingly hard, that it coll him an infinite deal of 
pains, and a journey of two hundred miles, to find out a 
ftone of fufllcient hardnefs to cut it, and whofe texture 
at the fame time was fine enough not to injure its furface. 
I have feen feveral of his finifhed metals ; they were in- 
deed perfectly hard andw’hite; but the kind of ftone with 
which he ground them he kept a fecret. 
After many experiments with various proportions of 
tin and copper, by gradually increafing the former, I at 
laft found that fourteen ounces and an half of grain-tin 
to two pounds of good Swedifh copper, made a beautiful 
white and very hard metal; fo hard indeed, that the 
ftones would but barely cut it, and wafhed emery on 
"brafs or tin but juft grind the furface without breaking 
it up ; whereas the proportion of tin being increafed by the 
addition of only another half ounce, the former incon- 
venience 
