What is the primary use of a Jacob's ladder?

A Jacob's ladder is primarily used to assist mariners while boarding the side of a vessel. This type of ladder is a series of rope rungs or steps that are suspended between two vertical lines, making it flexible and suitable for use off the side of a boat or ship. Its design allows for easy boarding and disembarking, especially when the vessel is at anchor or tied to a dock. The ladder hangs more vertically compared to traditional ladders, which can be particularly helpful in accommodating the movement of the boat with the tide or waves.

While it can be misleading, the other options do not accurately describe the function of a Jacob's ladder. For instance, securing cargo on deck and fishing from a boat are better achieved with different equipment and setups, while climbing up the mast generally requires a different type of rigging or ladder that is more stable and suited for vertical ascent. Thus, the key function of a Jacob's ladder is to provide a safe and efficient way for mariners to board a vessel, highlighting its importance in maritime operations.