Complex sentence: The men and horses began to weaken, so the decision was taken to leave eight men behind at Weymouth Bay, while Kennedy and four others continued north for several hundred miles to rendezvous with the ship "Ariel" to effect a rescue.
Split into simpler sentences: The men and horses began to weaken, so the decision was taken to leave eight men behind at Weymouth Bay, while Kennedy and four others continued north. There were still several hundred miles to go before any hope of a rendezvous with the ship "Ariel" to effect a rescue.