GRAVITATION AS AFFECTED BY TEMPERATURE. 
361 
One peculiarity of this form of suspension is that it never of itself comes to rest. 
For when the beam touches one tube at the end of a swing, it receives, by the tremors 
in the apparatus, enough energy to send it to the other end of its swing. This 
perpetual motion was counteracted by an arrestment (shown in the left side tube) 
worked by a magnet outside the vacuum. By this device the system was brought 
under control. 
This form of apparatus was abandoned on account of an unexpected form of 
attraction which caused the masses, m, m, to rush to the walls of the vacuum vessel 
and cling there indefinitely. These forces are very strong and existed despite the fact 
that there was a complete “ earthed ” system of aluminium tubing lining the vacuum 
vessel and that the suspension was “ earthed.” After separate investigation this effect 
was attributed to radiometric-pressure, which cannot be avoided in this form of vacuum 
vessel. 
In the final form of apparatus, to be next described, there are two distinct sets : 
(a) where the attracting masses are cylindrical; (b) where these are spherical. 
But as the latter type is more important, the following description applies to it 
throughout. 
VI. Final Experiments. 
1. General Description. —-In this form one may be said to have at last attained 
some mastery over the investigation, so that details will be given. A glass tube, AB, 
(figs. 6 and 7) is 1200 mm. long, 50 mm. bore. It is supported near the top by two 
strong glass tubes, C, D, issuing from opposite sides of the main tube (see fig. 6). 
The top and bottom of the main tube are closed and there is a window, W, in front. 
Tube C, finishes with a platinum wire sealed in for the purpose of earthing the inside 
of AB; while tube D is the connection to the pumping system (see fig. 6). 
2. The Support for the Vacuum Tube. —The glass tubes, C, D, rest on gimbals, 
CD, forming one axle, while a forked frame-work, V, of brass (fig. 8) carries the 
perpendicular axle, and itself is supported by being screwed to a stout beam. This 
beam, loaded by about 40 kilos, of lead weights, is carried by steel springs hung from 
the main scaffolding, and there are castor oil dash-pots to damp any chance vibrations 
which may be received from the ground. 
The lower end of tube AB, socketted into a copper sleeve, C u (fig. 8) is controlled by 
four setting screws mounted on a horizontal ring, ab. By this means AB can be set 
and held accurately vertical. 
3. The Torsion-head System. —A brass frame-work (fig. 6) fits firmly by three pairs 
of brass springs into the upper part of tube, AB. These six spring points are tipped 
with solder to avoid the well-known danger of hard metal scratching the inner surface 
of the glass tube. There are two magnets in the frame-work ; the lower one, m x , 
carries the suspension by pin, p, which turns freely, but without shake, in the two 
thoroughfare bearings shown. The upper magnet, m 2 , is mounted on a screw so that 
