VOL. I.] PHILOSOPHICAL TRANSACTIONS. 79 



with glass windows, and a hole at the top for the air, it will be more free from 

 dust, and irregular agitations. 



6. This instrument being accommodated with a light wheel and an index, 

 such as have been applied by Dr. Chr. Wren to weather glasses, and by Mr. 

 Hook to baroscopes, may be made to show minuter variations than other- 

 wise. 



7. And the length of the beam, and niceness of the balance, may make the 

 instrument still more exact. 



Though in some respects this statical baroscope be inferior to the mercurial ; 

 yet in others it has its own advantages and conveniences. As, first, it affords 

 an ocular proof that the falling and rising of the mercury depends upon the 

 varying weight of the atmosphere; since in this baroscope it cannot be pre- 

 tended that nfuga vacui, or a funiculus, is the cause of the changes we observe. 



2. It shows, not only that the air has weight, but heavier than some learned 

 men will allow ; since even the variation of weight in so small a quantity of air 

 as is but equal in bulk to an orange, is manifestly discoverable by such balances. 



3. This statical baroscope will often be more easily prepared than the other. 



4. The essential parts of the scale-baroscope may very easily in a little room be 

 carried any where, without the hazard of being spoiled or injured. 5. Mercu- 

 rial barometers contain air, more or less, but in the other, that consideration 

 does not take place. 6. It being possible to discover hydrostatically both the 

 size of the bubble and the contents of the cavity, as also the weight and di- 

 mensions of the glassy substance, we may easily discover by this instrument 

 this absolute and relative weight of the air. For, when the mercury is either 

 very high or very low, or at a medium height, bringing the scale-barometer to 

 an exact equilibrium, and observing when the mercury is risen or fallen just an 

 inch, or a half or fourth of an inch, &c. and putting in the like minute divisions 

 of a grain to the lighter scale, till you have again brought the balance to an ex- 

 act equilibrium, you may determine what weight in the statical baroscope an- 

 swers to the several altitudes of quicksilver. And if the balance be accommodated 

 with a divided arch, or a wheel and index, these observations will assist you for 

 the future to determine readily what the bubble has gained or lost in weight by 

 the change of the atmosphere's weight. 7 • By this statical instrument we may 

 be enabled to compare the mercurial baroscopes of several places, and to make 

 some estimate of the gravities of the air. As if, for instance, it is found by ob- 

 servation that the bubble weighed just a dram when the mercurial cylinder was 

 at the height of 294- inches ; and that the addition of the 1 6th part of a grain 

 is requisite to keep the bubble in an equilibrium, when the mercury is risen an 8th, 

 or any determinate part of an inch above the former height : and when in another 



