In choosing between two materials for submerged work practically equal in other respects, the heavier should be preferred, as by reason of its own weight it has greater power to resist the action of the waves and scour, and the decrease of its specific gravity by the weight of the bulk of water displaced is relatively not so large. With respect to the earthwork of canals and embankments constructed to hold water, each chapter of this book contains information relating to the promotion of the stability of the soil, and it is not here intended to refer to the most approved methods of construction, but only to name some points requiring attention. A barge or ordinary ship canal is usually placed at a shallow depth in the ground and meanders through a district, avoiding deep cuttings and heavy embankments, such an undertaking as the Suez, or the Panama, ship canals being altogether exceptional undertakings; and also the Manchester Ship Canal. Some of the most treacherous soils are named in Chapter II., but probably the worst earth in which a canal can be made is peat-bog land; the method of procedure is then different to that required in making a railway or a road, and in such soil the construction of a canal should be avoided, as it necessitates most experienced and skilful treatment and extensive drainage, which causes subsidence; difficult maintenance to preserve the channel and retain the water of navigation, control the drainage waters, and keep unimpaired the towing-path, which must be firmly covered throughout. Fine sand is also an unfavourable soil as it so readily becomes a quicksand, but almost all other earths usually met with can be so protected that failure should be an improbable contingency, except when the cuttings are of extraordinary depth, as on the Panama Canal; the chief danger being when there is considerable diversity, irregularity, fissuring, looseness, and upheaval of the soil, for varieties of almost every surface earth may be encountered, each with its own characteristics and behaviour when dry or water-charged, and requiring great attention, especially at the joints, and not a few separate treatment, not only of the soil but of the same earth in a cutting and in an embankment.