Theory of the Pneumatic Parador. 311 
A disk of card two inches in diameter, is placed horizontally at 
the upper end of the tube. Another card of the same size made 
slightly concave, is placed over the first. Let the whole be cov- 
ered with a glass receiver ; then let the air be removed, which of 
course can only quit the receiver by passing out through the brass 
tube ; when the exhaustion is carried to the utmost limit, let the 
air ie readmitted in a full gush through the brass tube, nig! the 
upper card will not be blown off. This experiment has been 
tried many times with various states of exhaustion, from a quarter 
of an inch of mercury to twenty inches, but in no case was the 
ecard blown off, or even agitated in the slightest degree ; therefore 
the upper card is not retained in its position by the pressure of 
the atmosphere.” 
Mr. Tomlinson’s failure to blow off the disk may have arisen 
from the concavity of the upper card, from not carrying the ex- 
haustion far enough in proportion to the size of the movable disk, 
or from the warping of the cards, which, owing to the evapora- 
tion of moisture usually contained in their substance even when 
apparently quite dry, is very apt to take place as the exhaustion 
proceeds. 'To obviate this difficulty, the fixed disk may be made 
of tinned sheet iron or brass, and the movable one of the same 
material, very thin and light, or of card recently dried by expo- 
sure to-heat, and pressed, in order that both disks may be perfectly 
plane and in contact throughout their whole extent, when applied 
to each other. For the convenience of suspending, during the 
process of exhaustion, the movable disk from the top of the re- 
ceiver, by means of a rod sliding through a collar of leathers, and 
disks are applied to each other, about an eighth of an inch into the 
tube. I repeated Mr. Tomlinson’s experiment with a very pow- 
erful air-pump, recently made for the University of Cambridge 
N. B. Chamberlain of this city. The passage by which air 
is admitted into the receiver is one eighth of an inch in diameter, 
Every thing having been adjusted as above described, the air was 
exhausted to such a degree as to reduce the mercury in a very 
accurate syphon gauge to within one tenth of an inch of a level 
in the two branches, indicating a residue in the receiver of only 
one three hundredth of the air naturally contained in it. By 
means of two stop-cocks the air was first admitted into the air pas- 
