HYDROGRAPH1C SURVEYS. 39 



should be so placed that their positions cannot be disturbed by passing craft, drift, 

 or other causes The observations should be made by reliable and careful persons 

 at the same hour each day at all stations, and in addition to the stage of water they 

 should note whether the river is rising or falling, and if an extreme of high or low water 

 has been reached during the 24-hour period the time and stage should be reported. 



Datum for Zeros. When the object of the gauge is to record heights of water for 

 the use of navigation, the zero is usually placed at the level of the lowest water known. 

 If it is to show the depth of water on a certain bar, shoal, or other point, usually the 

 point of least draught in a certain locality, its zero is fixed at the depth below the water 

 surface which will indicate the depth on the point in question. For instance, if the 

 depth is 3.6 feet on the head of Mustapha Island, in the Ohio River, the gauge at Park- 

 ersburg, 1 1 miles above, should read 3.6 feet, Mustapha being the governing bar of 

 that section of river. A pilot then knows at a glance whether he can proceed, because 

 the stage on the governing point indicates to him that all other points in the section 

 traversed have at least as much water as that shown by the gauge. 



Material for Gauges. Wood. The simplest form of gauge is a wooden board, 

 painted and properly graduated, but this is only of temporary duration. Their 

 renewal is necessary at frequent intervals because they are easily destroyed, and their 

 decay is certain. Their replacement is not always effected with absolute certainty 

 as to the zero being placed at the same height as before. They must be frequently 

 painted, and a derangement of their divisions may occur in this manner. A gauge 

 once properly established should remain without change or disturbance, and with 

 wooden gauges this is not practicable. 



Metal. Cast- and wrought-iron and steel gauges are in use and give fair satisfac- 

 tion. Those of cast iron usually have their figures and division lines raised, the body 

 of the gauge being painted white and the lines and figures black. Usually these gauges 

 are cast in sections from three to six feet in length, with suitable ears for fastening 

 them in position. These short sections facilitate the repairs which are sometimes 

 necessary, on account of breakages. On the upper Rhine the cast gauges have raised 

 enameled figures and divisions, and it is said the enamel does not scale off as it does 

 upon flat surfaces. 



Gauges of wrought iron and steel have been long in use, the figures and lines being 

 stamped and enameled. This form readily cleans itself and preserves a clearness of 

 color for a time, but the enamel is easily broken and scaled off, particularly on broad 

 surfaces where expansion plays a part, thus causing the figures and divisions to become 

 indistinct and the reading difficult and uncertain. This class of gauge is also rather 

 expensive. 



Stone. Gauges of stone are not uncommon in this country. They are sometimes 

 laid on the slope of the bank and form a walk. The stones are cut and laid care- 

 fully, precautions being taken to prevent undermining either on the edges or at the 

 foot. The division lines and figures are then located with a spirit-level and cut into 



