520 
Mr. Babbage on electrical 
ist. The various currents of air in the room could not pro- 
duce an almost uniform result. Considerable care was taken 
not to move about more than was requisite, and the experi- 
ments were generally made after the apparatus had been 
left, and the room shut up for some hours previous. In all 
but the earliest experiments the cylindrical millboard cases, 
in which the needles were placed, effectually excluded all 
currents of air exterior to them. 
2nd. The rotation of a current of air produced within the 
lower cylinder by the motion of the lamp, metal plate, and 
the wooden platform supporting them, would, if it produced 
any effect, tend to make the needle rotate in the same direc- 
tion. But we have seen from many experiments, that when 
a muslin screen is interposed, the rotation at the commence- 
ment is in the opposite direction. Also in Experiments 18 
and 19 there was no rotation given to the apparatus, and yet 
the needle advanced ; and in Experiment 16 a sheet of zinc 
prevented the air set in motion in the lower cylinder from 
communicating with that in the upper. 
3rd. That air driven through the screen was not the cause 
of these motions, appears from their taking place when the 
screen was impenetrable to it. In Experiment 9, where no 
electricity was employed, a rapid rotation was kept up dur- 
ing six minutes immediately under a gauze screen, and no 
motion was produced in the needle above it. 
4th. Perhaps the most probable of these suppositions is, 
that the motions result from the effect of currents of air 
which rise from the surface of the heated metal disc : these 
will cause other lateral currents, arising from the cold air 
flowing in to supply the place of that which has been heated, 
