Experiments and Qbfervationi . 67 
fometimes refting it upon pieces of cork, and then bring a 
powerful magnet near it* 
Examination of the Magnetical Properties of Brafs e 
A few years ago, being intent on making fome magnetic 
experiments, in which brafs was concerned, I ufed to examine 
firft whether the pieces of brafs had any magnetifm or not, and 
rejeded thofe pieces which had an evident degree of that 
power. In the courfe of thofe experiments I remember to have 
obferved, that thofe pieces of brafs which had been hammered 
were generally magnetic, and much more fo than others ; in 
confequence of which I made no ufe of hammered brafs in 
thofe experiments. But lately, having ordered a theodolite at 
a philofophical inftrument fhop, I particularly enjoined the 
workmen to try the brafs, both foft and hammered, before they 
worked it, and to make no ufe of that which had any mag- 
netifm. They found, that hammered brafs, even fuch as 
before the hammering had no magnetifm, could afterwards 
difturb the magnetic needle very fenfibly. Thefe obfervations 
induced me to make the following experiments. 
EXPERIMENT I. 
An oblong piece of brafs, weighing fomewhat lefs than 
half an ounce, being examined by prefenting every part of its 
furface to the fufpended needle, Ihewed no fign of magnetifm 
whatever. It was then hammered for about two minutes ; the 
confequence of which was, that it became magnetic fo far as 
to attraft either end of the needle from about a quarter of an 
inch. This fame piece of brafs being now put into the fire fo as 
K 2 to 
