midnight of May 19th
The forts on Staten Island and at Sandy Hook were ready with their artillery to attack anything tangible . Airplanes sped back and forth overhead . Troops were marching from outlying points -- lines of them coming in over all the bridges . By midnight of May 19th and 20th there were groups of ghosts visible everywhere about the city . They lurked in the buildings , permeating the solid walls , stalking through them , or down through the foundations ; they wandered upon invisible slopes of their own world , climbing up to gather in groups and hanging in mid-air over the city rooftops . In the Hudson River off Grant 's Tomb two or three hundred of the apparitions were seemingly encamped at a level below the river 's surface . And others were in the air over the waters of the upper bay .