In addition to the inclined bow waves, there is a similar system produced by the stern of a vessel, which is, however, much more difficult to detect. The other two wave-systems produced by a ship are generally called the transverse waves. There is a system of waves whose crest-lines are at right angles to the ship, and they may be seen in profile against the side of any ship or yacht as it moves along. These transverse waves are really due to the unequal pressures resulting from the distribution of the stream-lines delineating the movement of the water past the ship. If we return again to the consideration of the flow of a perfect fluid round an ovoid body, it will be remembered that it was shown that, in consequence of the fact that the stream-lines are wider apart near the bow and stern than they are opposite the middle part of the body, the pressure in the fluid was greater near the bow and stern than at the middle. When a body is not wholly submerged, but floats on the surface as does a ship, these excess pressures at the bow and stern reveal themselves by forcing up the water-surface opposite the ends of the vessel and lowering it opposite the middle. This may be seen on looking at any yacht in profile as it sails. The yacht appears to rest on two cross-waves, one at the bow and one at the stern, and midships the water is depressed (see Fig. 38). [Illustration: FIG. 38.] These waves move with the yacht. If the ship is a long one, then each of these waves gives rise to a wave-train; and on looking at a long ship in motion, it will be seen that, in addition to the inclined bow wave-system, there is a series of waves which are seen in profile against the hull. [Illustration: FIG. 39.] When a ship goes at a very high speed, as in the case of torpedo-boat destroyers, the bow of the vessel is generally forced right up on to the top of the front transverse waves, and the boat moves along with its nose entirely out of water (see Fig. 39). In fact, the boat is, so to speak, always going uphill, with its bows resting on the side of a wave which advances with it, and its stern followed by another wave, whilst behind it is left a continually lengthening trail of waves, which are produced by those which move with the boat.