The object of Jacketing is to prevent the loss of heat from the steam within the cylinder by radiation.
"Modern Machine-Shop Practice, Volumes I and II"
Joshua Rose
As a general rule, those from large engines will be more correct than from small ones, and those from high more correct than from low speeds, and in either case efficiently covering the steam pipes and Jacketing the cylinder, to prevent condensation, will improve the diagram.
"Modern Machine-Shop Practice, Volumes I and II"
Joshua Rose