A small shack (usually on wheels) put near a beach, used for changing into one’s bathing suit. A relic from the days when to be seen in one’s bathing suit by a member of the opposite gender was not considered proper.
Carroll could be making another reference to Charles Darwin. The HMS Beagle belonged to a class of ships which was described as “His Majesty’s bathing machines.” Also, Darwin visited Dr. James Gully’s Water Cure Establishment.
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