Part of this petition against grievances is concerned with the general policy of the king in matters of trade, as expressed in his "divers ordinances." But the greater part of it is directed against abuses which were hardly to be escaped at a time when navies were formed by impressing merchant ships. Corrupt or even only insolent and overbearing officers would abuse the power of impressment. Where those evil motives were not at work, there was still an all but irresistible temptation to "arrest" ships long before they were needed, since, if they were allowed to go on trading voyages, they would not be forthcoming later on. The king's officers were to be excused if they preferred to err on the safe side; but to the trader it was a grievous oppression, for he was deprived of the means of earning profits, while remaining liable to be taxed in order to provide the king with a revenue for the support of the war. In later ages the impossibility of combining the qualities of money-earning merchant ships and of fighting ships, which should be always available for war, had much to do with the formation of regular military fleets. In the seventeenth century the State took, first, to hiring vessels for long periods, and manning them itself; then, as the need of a special class of vessel grew with the development of artillery, to building for itself. In the Middle Ages no State was yet rich enough to maintain for long together a great and costly naval force. Thus it was necessary to rely on impressed vessels, which could only temporarily be withdrawn from commerce. Fleets formed in such a way bore an inevitable resemblance to armies composed of farmers, townsmen, or mountain clans collected for a single foray or battle, and always liable to dissolution on the approach of harvest, or even under the influence of the occasional soldier's not unnatural desire to put his booty in a safe place. In the great Civil War of the seventeenth century, the Parliament discovered that the London trainbands, though capable of good marching and hard fighting, as they proved during the relief of Gloucester and at the battle of Newbury, soon grew eager to be back to their shops, and mutinous if they were kept out for what seemed to them an undue period. So it must always be with a citizen force. Since the mediæval navy was largely of that description, it suffered from recurrent lapses of strength, and was peculiarly liable to total collapse when the country was overwrought by the strain of long war.