>According to his nephew,[20] 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").[15] 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.
Spoilering for there are some future facts from here on out.