26 U. S. COAST AND GEODETIC SURVEY 
Box Gage 
Where conditions make readings on a plain staff difficult, a box gage may be used. 
Essentially this consists of a float that rises and falls in a vertical box to which the tide 
has access. The box may be made of 1-inch boards 12 inches wide, the bottom being 
closed, except for a hole about 1 inch in diameter through which the tide has access 
and which reduces the wave motion in the float box very considerably. A convenient 
form of float for a box gage is a copper cylinder about 8 inches in diameter and 2 or 3 
inches high with tapering top and bottom sections. 
Various means may be used for determining the rise and fall of the float in a box 
gage. Where the range of the tide is moderate, a light wooden rod graduated to feet 
and tenths may be secured to the top of the float and at a convenient point above the 
top of the float box the rod made to pass through a metal ring secured in such wise that 
the axis of the rod is vertical. ‘The metal ring serves the further purpose of furnishing 
a reference point for reading the height of the tide. It is to be noted that it is necessary 
to graduate the rod with the numbers increasing from top downward, in order that 
the heights of the tide as read on the rod may be direct and not inverted. 
Where the range of the tide, or the distance from the top of the box gage to the 
surface of the water, is considerable, a graduated steel or phosphor-bronze tape 1s more 
‘ convenient than a rod. In this case the lower end of the tape is attached to the float, 
and the upper end is made to pass over a fixed pulley. To keep the tape in tension, 
a weight is attached to its upper extremity, and for a reading point for measuring the 
rise and fall of the float, the tape may be made to pass a metal ring fixed at a convenient 
distance from the top of the float box, or a board may be fixed vertically near the tape 
and a reading line marked on the board. 
The relation of the zero of a box gage to fixed bench marks on shore may be deter- 
mined in two different ways. In the first method simultaneous readings of the box 
gage and a fixed tide staff are made and the relation of the zeros derived. The eleva- 
tions of the bench marks above the zero of the fixed tide staff are then determined in 
the usual way, and the relation of the zero of the box gage to the bench marks is then 
determined through the difference of the zeros of the tide staff and box gage. In this 
method care must be taken to have the fixed tide staff near the box gage, so that at any 
instant the height of the tide is the same in the box gage as on the tide staff. 
Another method of determining the relation of the zero of the box gage to fixed 
bench marks on shore consists in determining the elevations of the bench marks relative 
to the reading point and adding the length of the float rod or tape from the zero gradua- 
tion to the line of flotation of the float. The first part of this operation is accomplished 
in the usual manner with the spirit level. The second part is accomplished by float- 
ing the float with rod or tape attached in a pan of water, care being taken to have the 
density of water in the pan the same as that in the float box, and measuring the distance 
between the zero of the rod or tape and the line of flotation of the float. 
Automatic Tide Gages 
Where the tide observations are to cover a period of several months, the automatic 
or self-recording tide gage is the more satisfactory. Various forms of automatic tide 
gages are on the market, some of these tracing a continuous curve and others printing the 
