Complex sentence: This put the chronometers at risk and the disturbance of moving them affected their accuracy; this was another reason for survey ships to carry a large quantity--most of them could be kept permanently in a safe, well-cushioned place on the main vessel.
Split into simpler sentences: This put the chronometers at risk and the disturbance of moving them affected their accuracy. This was another reason for survey ships to carry a large quantity--most of them could be kept permanently in a safe, well-cushioned place on the main vessel.