(235)
tagious to rarify and drive out the Air as much as is possible, because in such seal'd Bubles the Air it self (as I have elsewhere shewn) has a weight; yet this advantage countervails not the inconvenience of being obliged to increase the weight of the Glass, which when it includes highly ratified Air, if it be not somewhat strong, will be broken by the pressure of the External Air, as I have sufficiently tryed.
Thirdly, I would have tryed, whether the Dryness and Moisture of the Air would in any measure have alter'd the weight of the Buble, as well as the Variation of Gravity produced in the Atmosphere by other causes; but the extraordinarily constant absence of Fogs, kept me from making Observations of this kind; save that one morning early, being told of a mist, I sent to see (being my self in bed) whether it made the Air so heavy as to buoy up the Buble; but did not learn, that that mist had any sensible operation on it.
Fourthly, By reason of the difficulties and casualties, that may happen about the procuring and preferring such large and light Bubles, as I have been lately mentioning; it may in some cases prove a convenience to be inform'd; That I have sometimes, instead of one sufficiently large Buble, made use of two, that were smaller. And, though a single Buble of competent bigness be much preferable, by reason, that a far less quantity and weight of Glass is requisite to comprise an equal capacity, when the Glass is blown into a single Buble, than when it is divided into two; yet I found, that the employing of two instead of one, did not so ill answer my expectations, but that they may for a need serve the turn instead of the other; than which they are more easier to be procured: And if the Ballance be strong enough to bear so much Glass, without being injur'd; by employing two or a greater number of large Bubles, the effect may be more conspicuous, than if only a single Buble (though a very good one) were employed.
This instrument may be much improved by divers Accommodations. As
First, There may be fitted to the Ansa (or Checks of the Ballance) an Arch (of a Circle) divided into 15. or 20. deg. (more or less, according to the goodness of the Ballance) that the Cock resting over against these Divisions, may readily
