300 Mr. mudge on the ConJlruSUon 
tharpnefs in the image as there were defective points of. 
reflection in the metal. 
Betides the trouble of a great number of experiments, 
in order to get rid of this mifchief, and to afcertain the 
eaufe to which it was owing, there was this additional 
inconvenience attending it, viz . that the fault was not 
difcovered, as was obferved before, till a great deal of 
trouble had been taken in grinding and even polifhing 
the metal, the whole of which was rendered ufelefs by 
the mortifying difcovery of this defeat. 
I was extricated at laft from this difficulty, and in fome 
meafure by accidept. Having one day made a great num- 
ber of experiments, and having melted down all the good 
copper I had or could procure ; though puzzled and fa- 
tigued, yet not caring to give it up, I recollected that I 
had fome metal which was referved out of curiofity, 
and was a part of one the bells of St. Andrew’s which 
had been re-caft. Expecting, however, very little from 
this grots and uncertain compolition, I was neverthelefs 
determined to fee what could be made of it by enriching 
the compolition with a little freffi tin. Accordingly 
catting a metal with it, it turned out perfectly free from 
pores, and in every refpeCt as fine a metal as ever I taw. 
I could not at fir ft conceive to what this fuccefs was 
owing ; but at laft I hit upon the real eaufe of that defeCt, 
c which 
