508 
ME. W. CROOKES ON ATTRACTION AND 
owing to its expanding by heat. A gridiron beam of zinc and iron or of zinc and glass, 
and a beam formed of two reversed thermometers, their bulbs forming the gravitating 
masses, were thought of, so as to have self-compensation for changes of temperature ; 
but after a few experiments the weight of such beams was found to be an insuperable 
objection, even had the difficulties of adjustment been overcome. Altogether glass 
seemed to offer most advantages, as being sufficiently rigid, whilst its low conducting- 
power for heat rendered it little liable to introduce errors from expansion. 
A straight glass beam was drawn from a piece of rectangular glass tube, and it was 
fitted with needle-point centres and brass balls at each end, similar, to the plan adopted 
with the brass and charcoal beams. This was fitted in the small brass case, and many 
experiments were tried with it. The results were still anomalous, the apparent action 
of heat being sometimes in one direction and sometimes in another (49). 
Greater delicacy was still required, and the brass balls at the end of the beam were 
accordingly replaced with magnesium balls ; and instead of enclosing the balance in 
the brass case, I sealed it up, after exhaustion, in a glass tube. With this a large 
number of experiments were tried ; but as the subsequent results, after I had discovered 
one of the laws governing the phenomena, were much more satisfactory, I forbear from 
occupying space in describing these preliminary experiments. 
23. Afterwards I experimented successively with a glass beam having no special 
weight at the ends, and with the same beam with terminal knobs of glass fused on ; 
this appeared to increase the delicacy somewhat, but the weight was still too great to 
allow of accurate results being obtained ; and I finally adopted straw as the material for 
the beam, varying the gravitating masses at the end as experience dictated. Straw 
possesses many advantages : it is exceedingly light yet rigid ; it dries easily, and 
evolves no vapour in a vacuum ; moreover it is not likely to introduce errors by altering 
in shape under the influence of the moderate degrees of temperature to which it is 
subjected in these experiments. 
24. The method of supporting the straw beam in the centre, so as to secure the 
maximum sensitiveness without the liability to get out of order, was difficult at first. 
After trying suspension by fibres of cocoon-silk from a glass frame, suspension on a 
fine glass axis resting on thin glass rods, and many other devices, I finally adopted the 
following mode of support. 
The pointed half of a small sharp needle is broken off about half a millimetre shorter 
than the internal diameter of the glass tube ; the blunt end is then ground very sharp 
on Arkansas stone. The straw, about 7 inches long, having its gravitating masses at 
the ends, is then balanced on a knife-edge so as to let it roll over to a stable position 
nd to find its centre ; and the needle is then run through it at right angles, at such a 
distance above the horizontal centre of the straw that the centre of gravity of the Avhole 
system is a little below the centre of suspension. The beam being slipped into the 
glass tube (sealed at one end), the needle is supported very delicately against the sides 
of the glass by its points, and with the least possible amount of friction. It is best now 
