may be roJied on, or that it may 'be better tryed, .if mine 
be found deteftive. 
- vT-be iirft Irift4un3€nt was an Air-Pr/mp of Mr Hhw^esbee's, 
a perfdn merirorioufly known to the R. Society, who I 
doubt not is the bcft Maker of thofe kinds of Inftru^ients 
in the Worlds and his Air-Pupps exceed others in, the 
Eafe gt being pumpeds aiid^in the Certainty o£ their^^va- 
cuation^ which may be known to a great nicety. 
The next Inftrument was the aforefaid Small Movimnty 
with a Pendulum of about lo inches, that vibrateth Half-- 
feconds^ and is driven by the ?,oj^v pi a Spring. This 
Inftrument I thought commodious, not only for being 
cafily fitted' with a Receiver, b^t alio for vibrating Halt- 
feconds very nicely, arid alfo becaule its Vibrations are 
equal, not fome large, fome fhorter. 
The laft Inftrument was a very well regulated 
that vibrateth with as much 
exaftnefs'as moft do.^ " V ''" ^" ' . 
Being thus accoutered, the Refult of many repeated . 
; Experiments, day after day, was. That ( as before ) in 
Vacuo the Vibrations were always larger, than in tfce Re- 
ceiver unexHaufted. At the firft, when my little Move- 
ment was newly cleaned^ Vibrations were above 
of an Iriich larger than in the free Air. But after v;^ards (T 
.fuppofe frpm fome of the fouled -Oyl of the Pump fpirtled 
on the Wheels, in letting in the Air, whereby the force ^ 
of the Spriag on the Pallets was , blunted, from hence 
fay) as the Vibratioqs iri^the uriexhaufted Receiver were 
a little contrafted, fo in the Receiver exhaufted they were 
more contraded, and only about q^ 35^of an inch larger 
than in the Free Air- hn^ 
The Alteration in Time^ which this difference of the VK 
brltions produced, .was conftantly only abQut ,^8 f^conds in 
an Hbnr flower, in me Receiver exhaufted^. jthgi^ i§,it .unex*^ . 
hailed; For^ifjin 4, .5 or nio^^ .l^opi^ -agoing,: I thfij^tjMpc:! 
Pemiulunis did hot \uj 2l (^xx%^^ in the open 
Air, 
