358 
DR. P. E. SHAW ON THE NEWTONIAN CONSTANT OF 
effect would be hard to disentangle from the true gravitative temperature effect as 
they would both be cyclical. 
2. We now come to the action of the gas remanent in the vacuum tube. The 
mechanical effects which this produces on the delicate torsion system are very complex. 
There are four distinct influences at work, viz., convection, radiometric pressure, 
discharge of occluded gases, damping :— 
(a) Convection .—This causes great trouble in the ordinary gravitation experiments ; 
here, where temperature is greatly changed, it would make measurements at normal 
pressures useless. Hence we use a vacuum. In the highest attainable vacuum, 
convection would quite vanish. We even find that for a poor vacuum of about 
1 mm. it produces little disturbance in a properly screened apparatus. 
P&B 
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Fig. 3. Gas troubles depending on pressure in a vacuum. 
(b) Radiometric pressure is a maximum at a pressure of 30//. of mercury 
(/ul = l/lOOO mm.). It has been detected at a pressure of 0‘015/x, but it is negligible 
for lower pressures and probably for pressure higher than 1 mm. with due precautions. 
(c) The evolution of occluded gas from the wall of the vacuum vessel and from 
the torsion system should be negligible, even in a high vacuum, after long continued 
exhaustion with heat, as in these experiments. 
{d) In a high vacuum the damping of the torsion system of great period is 
perceptible, though slight. But it is essential that the tremors continually received by 
the torsion system from the ground should be quickly damped out if accurate telescope 
readings are to be taken. The present research has been carried out in the heart of 
Nottingham, with a trunk railway system a quarter of a mile away, so that even in 
the dead of night it has been impossible to use high vacuum, except for the particular 
