I was out riding with Miss. P. An uncomfortable object is in the seat and I change my position and she comments on it. We stop and she goes in a place on an errand but the car continues rolling to the end of the wharf. I try to break it but it doesn't stay still. Man comes to help. He says "You'd better get out and let me get in and fix it." He goes up the incline, the car disappears. I rush up to the top and ask doorman which way the car went, but can't get a thing out of him. Rush upstairs and tell directors in office, then down to tell Miss. P., then return. Man inquires me what I know and tells me they have the loop surrounded. I ask him if they have the license number and take him to meet Miss. P. The car was white.