24 U. S. COAST AND GEODETIC SURVEY . 
short time, the Coast and Geodetic Survey makes use of vitrified scales. These are 
made by baking a vitrified coating on wrought iron strips. The strips are in 3-foot 
sections about 2% inches wide, the sections being so graduated that when placed end to 
end they form a single continuous scale. They may be fastened to a suitable board or 
piece of timber to form the tide staff. 
Readings on the staff should be recorded every half hour or hour, except near the 
times of high and low water, when the readings should be made every 15 minutes or even 
more frequently. Continuous observations covering both day and night are most 
satisfactory, but, where this is not feasible, daylight observations over a period of 13 
consecutive hours every day should be made. 
Bench Marks 
The zero of the tide staff should be connected by spirit levels with at least three 
good bench marks. This will make possible the replacing of the tide staff at the same - 
elevation during the progress of the observations, should it become destroyed or should 
its elevation be changed by accident. The bench marks will also serve the further 
purpose of preserving for future use the datum planes that are determined from the 
tidal observations. The bench marks should be placed at some distance from each 
other so that they are all not likely to be destroyed by a common cause. 
It is the practice of the Coast and Geodetic Survey to establish and maintain at each 
tidal station not less than one standard disk bench mark for each year of observations 
up to 10 years, with a minimum of five such marks for a series of one year in length and 
a minimum of three for a series less than a year. Three of these bench marks are located 
within a short leveling distance of the tide staff while the remainder are more widely 
distributed to insure against loss from a common cause. Care is taken to avoid locating 
the bench marks on filled-in ground. 
The qualities that distinguish a good bench mark are freedom from likelihood of 
change in elevation and ease of finding and identification. Disk bench marks fulfill 
these requirements well. The standard tidal bench mark of the Coast and Geodetic 
Survey consists of a brass disk about-3 inches in diameter, with a shank about 2} inches 
long for insertion into a building or other substantial support, and carries the inscription 
shown in Figure 10. 
Permanent and substantial buildings afford the best locations for setting the disk 
bench marks. The bench mark is countersunk, with its face flush with the surface 
of the part of the building into which it is set, and is securely cemented in, so that 
it will effectively resist extraction, rotation, or change of elevation. If the wall of a 
building is used, the bench mark should be set with its central line horizontal, for it is the 
elevation of this central line that is taken as the elevation of the bench mark. If a 
suitable location on a building is found which permits the disk bench mark to be set with 
its face horizontal, it is to be preferred, since this position is a more convenient one for 
placing a leveling rod. 
A boulder or a ledge of rock makes a very satisfactory location for a bench mark. 
A good foundation for a bench mark is also furnished by a mass of concrete with its 
upper surface slightly above the level of the ground, about 2 feet square on the bottom 
and 1 foot square on top. The mass should extend not less than 3 feet below the surface 
of the ground; but in localities of severe winters the depth should be sufficient to with- 
