The only obvious risk of failure lay in the chance that, my calculations failing or being upset, I might reach the desired point too soon or too late.

In either case, I should be dangerously far from Mars, beyond his orbit or within it, at the time when I should come into a line with him and the Sun; or, again, putting the same mischance in another form, behind him or before him when I attained his orbit.

But I trusted to daily observation of his position, and verification of my "dead reckoning" thereby, to find out any such danger in time to avert it.
The displacement of the Earth on the Sun's face proved it to be necessary that the apergic current should be directed against the latter in order to govern my course as I desired, and to recover the ground I had lost in respect to the orbital motion.

I hoped for a moment that this change in the action of the force would settle a problem we had never been able to determine.

Our experiments proved that apergy acts in a straight line when once collected in and directed along a conductor, and does not radiate, like other forces, from a centre in all directions.