78 PHILOSOPHICAL TRANSACTIONS. [aNNO l666. 



And first, if the ground on which I went in framing this baroscope be 

 demanded, the answer in short may be ; 1 . That, though the glass bubble and 

 the glass-counterpoise, at the time of their first being weighed, be in the air, 

 exactly of the same weight; yet they are very different in bulk; the bubble be- 

 ing perhaps a hundred or two hundred times larger than the metalline coun- 

 terpoise. 2. That, according to hydrostatical laws, if two bodies of equal gra- 

 vity, but unequal bulk, come to be weighed in another medium, they will be 

 no longer equiponderant. If the new medium be heavier, the greater body, 

 being specifically lighter, will lose more of its weight than the less and more 

 compact one; but if the new medium be lighter than the first, then the 

 larger body will outweigh the smaller : and this disparity, arising from the 

 change of mediums, will be so much the greater, by how much the greater in- 

 equality of bulk there is between the bodies. 3. Comparing these two together, 

 I considered that it would be all one, as to the effect, whether the bodies were 

 weighed in mediums of different gravity, or in the same medium, in case its 

 specific gravity were subject to considerable alterations. 



Though a single bubble of competent size 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 bubble, than when it is divided into two ; 

 yet if the balance be strong enough to bear so much glass, without being hurt, 

 by employing two or a greater number of large bubbles, the effect may be more 

 conspicuous than if only a single bubble were employed. 



This instrument may be much improved by divers accommodations. As 



1 . There may be fitted to the cheeks of the balance an arch of a circle, 

 divided into 15 or 20 degrees, more or less according to the goodness of the 

 balance, that the cock pointing to these divisions may readily, and without 

 calculation, show the quantity of the angle, by which, when the scales incline 

 either way, the cock declines from the perpendicular, and the beam from its 

 horizontal parallelism. 



2. Instead of the ordinary counterpoise of brass, one of gold may be em- 

 ployed, or at least of lead, whereof the latter being of equal weight with brass, 

 is much less in bulk, and the former amounts not to half its magnitude. 



3. Those parts of the balance made of copper or brass will be less subject to 

 rust than steel ; which yet, if well hardened and polished, may last a good while. 



4. Instead of the scales, the bubble maybe hung at one end of the beam, 

 and only a counterpoise to it at the other, that the beam may not be burdened 

 with unnecessary weight. 



5. The whole instrument, if placed in a small frame, like a square lantern 



