ON INSTRUMENTS FROM ITALY. 137 



on their constitution : " Either on account of their slippery smoothness, 

 or on account of the rotundity of their extremely small bodies or for 

 some other figure which they may take, particularly inclined to motion, 

 being just equilibrated, as soon as they are pressed they immediately 

 give way on all sides and scatter themselves . . ." so that it would 

 not be possible in our days to discuss their fundamental property 

 any better. And another important law is added further on : " In all 

 liquids this force of the pressure of the air is admirably shown, par- 

 ticularly when they are caught in some place where they have on one 

 portion of their superficies an empty, or nearly empty, space into 

 which they can withdraw. Since, in this case, being pressed on one 

 side by contiguous air, which in its turn is pressed down by so many 

 miles of amassed atmosphere, and on the other side, where there is 

 no obstacle, are confined by the void which has no weight at all, they 

 rise until the weight of the liquid raised up is equal to the weight of 

 the air pressing on the other side." It is afterwards observed that by 

 vacuum or void is meant that the air alone should be excluded, and 

 not light, or heat, or ether. 



We are indebted to the Accademia del Cimento for the usual ex- 

 periment which is made under the bell of the pneumatic machine, with 

 the bladder that swells by exhausting the air, and also swells when it 

 is forced back into it ; the second of those two barometers, the one 

 with the little well (pozzetto] outside, the other shut up in a bell in 

 which the vacuum is obtained, and which was invented to prove that 

 it is really the pressure of the air that holds up the mercury, and which 

 was afterwards altered by immerging the barometer in a vessel full of 

 water, so that the mercury was found to rise above the usual braccio 

 and ^ by T \ of the height of the water in the vessel, the level being 

 taken from that of the mercury in the little well (pozzetto). 



Then availing themselves of Vivianf s mercurial pneumatic machine, 

 they instituted a number of experiments in vacuo, to see whether their 

 operations prove contrary to, or in any way different from, those which 

 manifest themselves when surrounded by air. They accordingly experi- 

 mented on the spherical shape of the drops independent of the pressure 

 of the air ; on the heat and the cold which cause the rise or fall of the 

 barometrical column ; on the reflection of images in lenses, attributed 

 by Kepler to the air ; on the attraction of amber in vacuo ; on the 





