According to his nephew, during this period, he wrote his first book (perhaps in winter quarters when more spare time was available), a work on the use of missiles on horseback, "De Jaculatione Equestri" ("On the Use of the Dart by Cavalry"). It has not survived, but in "Natural History", he seems to reveal at least part of its content, using the movements of the horse to assist the javelin-man in throwing missiles while astride its back. During this period, he also dreamed that the spirit of Drusus Nero begged him to save his memory from oblivion. The dream prompted Pliny to begin forthwith a history of all the wars between the Romans and the Germans, which he did not complete for some years.