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Degrees, on the fame Point; which was a very fmall 
fewing Needle. 
The Number of Vibrations with the Agatp Cap, 
on the firft Trial, were 39, then 37, then 39 again; 
with one of the glafs Caps it made 2.3, and then 20. 
This Difference from the Agate Cap was fo great, 
that I concluded the Point muft be damaged, and 
therefore chofe a finer; on which the fame glafs 
Cap made 41 Vibrations; then 43 ; and another glafs 
Cap made 47, and the next time 43. But the Agate 
Cap with this Point made yi, J 7 , and y8 Vibrations. 
The unpolifhed glafs Cap performed much the fame 
with the others. I had two of them polilhed again, 
by Mr. Smeaton ; and in Company with him re- 
peated the fame Experiments; but with no better 
Succefs. The Agate Cap made always many more 
Vibrations than the glafs one ; and generally with 
the latter the Number diminifhed by repeated Trials;, 
whereas with the Agate Cap it ufually increafed. 
Thefc Experiments made me lay afide the Thoughts 
of glafs Caps,, and put me upon thinking how Agate, 
ones might be made with as little Expence as pol- 
fible. 
With this View I got a Cap turned of Ivory, in 
fuch a manner as to receive a fmall Bit of Agate at 
the Top. This being ground concave, and polifhed 
on that Side, where it formed the Apex of the hol- 
low Cone in the Cap, was capable of anfwering the 
Purpofc as well as if the Whole had been Agate, 
and was much lighter.. Thefe Caps may be made 
cheap enough for common Ufe; and, if good at 
firft, cannot eafily be impaired. 
Eojc 
