A PENDULUM FOR THE REDUCTION TO A VACUUM. 
403 
subject to be otherwise of importance in a scientific point of view, I resolved 
to devote some time to its examination: and, for this purpose, caused a vacuum 
apparatus to be fitted up at my own house, where I could pursue the subject 
at leisure. This vacuum apparatus is very different, in its form and con¬ 
struction, from that which is erected at the Royal Observatory at Greenwich, 
and described by Captain Sabine in the Philosophical Transactions for 1829, 
page 207- It consists of a brass cylindrical tube 
about five feet long and six inches and a half in 
diameter, rounded at the bottom, and soldered 
at the top to a thick iron frame, on which the 
agate planes rest. This frame is firmly screwed 
and fastened to solid mahogany beams which 
are securely wedged between two fourteen-inch 
walls in the corner of a room, which is remark¬ 
able for preserving an uniformity of temperature, 
during the day, throughout the different seasons 
of the year. The upper surface of this iron frame 
is ground perfectly plane, and is surmounted with 
a moveable glass top, in the manner described 
by Captain Sabine. The brass tube has two small 
openings, or windows (cut on opposite sides) at 
a proper distance from the top, which are cover¬ 
ed with plate glass, for the purpose of observing 
the arc of vibration and the coincidences with 
the clock, which is placed behind. The lower 
part of the tube is secured also by cross beams, in order to prevent any lateral 
motion in the tube itself, during the vibrations of the pendulum. As the 
whole of the experiments about to be related, are comparative only, it will be 
unnecessary to enter more minutely into a description of this apparatus ; the 
form and construction of which will be best understood from the annexed 
sketch. The flexible metallic pipe, communicating with an air pump, enters 
the tube immediately under the upper beams: and a brass wire, passing 
through a stuffing-box, for the purpose of setting off the pendulum at any given 
arc, enters the tube just below the glass window. 
