Generations of scientists had uncovered the details she needed to quantify the effect: she was mining the entire intellectual legacy of the Peerless. Agata hadn’t used half of these results since she was a student, but though her memory of some of the formulas was hazy – and she was afraid to consult the photonic library lest she alert someone in authority to her sudden change of interests – she discovered that she still had all her old paper textbooks at the bottom of a cupboard, not yet so insect-damaged as to be indecipherable.

Without access to the tubes themselves she couldn’t hope for a precise answer, but she could sketch the limits of what was physically possible. In the worst case, time had already run out: if the tubes were large enough and the seals sufficiently tight, it could take half a year to infuse enough modified air into them to corrupt the signals.

In the best case, it would take close to three stints. So she had, at most, two stints to alter the composition of the mountain’s atmosphere sufficiently to get the process started.

Agata rechecked the numbers, but they did not improve. She sat at her desk with her tattered books around her, bewildered but refusing to be cowed. The ancestors had spoken to her; she was joined to them across the disruption, across the generations yet to be born. The cosmos had no choice but to find a sequence of events that filled the gap and completed that connection, and it could not come out of nowhere. The right plan had to lie within her, just waiting to unfurl.

29

Ramiro was beginning to wish that they’d put cameras on the occulters. The extra transmissions needed to send back the images might have increased the chance of detection, but it would have been worth it just to have an objective version of the rendezvous with the cache in front of him, to take the place of the pictures in his head.

First, the occulter had to release itself from the rock, unwinding the drills and letting itself fall into the void. Then the air jets had to catch it and send it swooping back towards the slopes, approaching the cache with just the right speed at just the right angle. Two hooks on strings hung down from the cache, each one an open half-circle crossed by a vertical trigger about a third of the way in; the arms of the occulter needed to enter those half-circles and strike the triggers to send the second, spring-loaded halves sliding around to enclose them. Then the occulter had to move away, dragging the cache almost horizontally across the rock, unrolling the adhesive resin that was holding it in place against the vertical tug of its centrifugal weight.

Tarquinia interrupted his brooding. ‘Relax,’ she said. ‘Or add up the navigational tolerances again, if you want reassurance. We can hit the hooks, I’m sure of it.’