468 
MR. W. CROOKES OK THE SUPPOSED 
I consider, perfectly explained.in the paper by the “ molecular bombardment” theory, 
and I, therefore, am justified in assuming that the negative rotations in M. Thore’s 
apparatus will equally well be explained by the same theory. 
Addendum. 
(Added May 24, 1887.) 
I sent M. Thore a detailed account of my experiments, asking him to favour me 
with any comments or remarks he might wish to make, and offering to communicate 
them, if desirable, to the Royal Society. I have just received a long communication, 
partly printed and part in MS., in which he describes many fresh experiments, and 
adduces arguments to show that my dynamical explanation is not sufficient to account 
for more than a few of the facts he describes, and saying that he “ persists in still 
believing that this force emanates from the observer, or else that the observer is the 
indispensable intermediary for its manifestation/' 
The experiments are numerous, and are devised with great ingenuity. It is 
impossible in the space of a brief abstract to do more than refer to a few of the 
principal facts here brought forward. M. Thore commences by objecting to my 
having experimented in an enclosed space, saying that he always operates in free ah’. 
He thinks that enclosure may almost or quite suppress his force. To this I can reply 
that I have myself verified nearly all M. Thore’s facts of rotation (including those 
just now communicated) when working in the free air of a large room, and it was only 
when I found the delicacy of the observations was impeded by draughts and currents 
that I put screens round the apparatus. I have not found glass screens interfere 
materially with any of the rotations. M. Thore now says that it is necessary to hold 
the pillar or the exciting body in contact with the hand during the whole duration 
of the experiment. I was not aware that importance was attached to this point, 
but I have since repeated many of my former observations, holding the pillar in 
the hand. The results are certainly stronger; but the extra heat imparted to the 
apparatus is, in my opinion, sufficient to account for this. M. Thore brings forward 
many new and ingeniously devised experiments to prove that heat cannot be 
considered the cause of the movement. He exposes the instrument to the full sun, 
and then brings it into a cool dark room ; he suspends it over boiling water ; he 
places a large block of ice between the cylinder and the observer; he similarly 
interposes metallic vessels full of boiling water between the cylinder and observer 
(the observer not moving from his place in front), and he tries the experiment in a 
hot chamber, alternately moist and dry, without finding the regularity of the movements 
interfered with. I have tried most of these, and obtained results corroborating 
M. Thore’s, but I have also tried the experiment of quietly bringing near to the 
stationary cylinder a bottle of hot water, and observing the movement from a safe 
