In the meanwhile, Fay was taking care of the other necessary elements of his scheme. Besides the mechanism of the bomb, he had to become familiar with the shipping in the port of New York, and he had to get the explosive with which to charge the bomb. For the former purpose he and Scholz bought a motor boat--a 28-footer--and in this they cruised about New York harbour at odd times, studying the docks at which ships were being loaded with supplies for the Allies and calculating the best means and time for placing the bombs on the rudder posts of these ships. Fay finally determined by experience that between two and three o’clock in the morning was the best time. The watchmen on board the ships were at that hour most likely to be asleep or the night dark enough so that he could work in safety. He made some actual experiments in fastening the empty tanks to the rudder posts, and found that it was perfectly easy to do so. His scheme was to fasten them just above the water line on a ship while it was light, so that when it was loaded they were submerged and all possibility of detection was removed. The getting of explosives was, however, the most difficult part of Fay’s undertaking. This was true not only because he was here most likely to arouse suspicion, but also because of his relative lack of knowledge of the thing he was dealing with. He did know enough, however, to begin his search for explosives in the least suspicious field, and it was only as he became ambitious to produce a more powerful effect that he came to grief. The material he decided to use at first was chlorate of potash. This substance in itself is so harmless that it is an ingredient of tooth powders and is used commonly in other ways. When, however, it is mixed with any substance high in carbons, such as sugar, sulphur, charcoal, or kerosene, it becomes an explosive of considerable power. Fay set about to get some of the chlorate.