54 XJ. S. COAST AND GEODETIC SURVEY 



sounding will not be done at a considerable distance from a tide 

 station. 



175. Effect of wind. — In large bays of comparatively shallow depth 

 and with small range of tide, and in broad stretches of rivers or 

 along shores where the water is shoal, the wind has considerable 

 effect on the time and height of the tide. In such places under 

 unfavorable weather conditions the state of the tide at moderate 

 distances from a tide gage may be quite different from that at the gage, 

 particularly if differently exposed to the wind direction. Sounding 

 at some distance from a tide gage under such conditions will result 

 in failure of sounding lines to cross, at times by several feet. 



176. Where such conditions prevail, a tide staff or portable gage 

 should be established in the immediate vicinity of the work for the 

 reduction of soundings and connected with the central or control 

 gage by simultaneous observations made during normal weather 

 conditions. If a portable gage is established at the auxiliary station, 

 2 or 3 days of simultaneous observations should be obtained. If a 

 plain staff is established at the auxiliary station, the observations for 

 connecting with the central gage need cover only those hours during 

 which sounding dependent upon the auxiliary gage is being done, 

 except that several high and several low waters should be included, 

 preferably a complete range of tide on each day that the staff is 

 used. 



177. When surveying shallow bodies of water, such as the sounds of 

 North Carolina, where the range of tide is small and fluctuations 

 due to metierological conditions considerable, it will be necessary 

 to confine the sounding work of any day during periods of heavy 

 winds to an area in the vicinity of a tide gage or, if sounding on 

 long lines, to have sufficient auxiliary tide stations in actual oper- 

 ation scattered over the area to furnish correct reducers for the 

 soundings at different positions along the sounding lines. 



ESTABLISHMENT OF SECONDARY TIDE STATION 



178. Certain precautions which are required when establishing a 

 primary tide station because of its more or less permanent character 

 are unnecessary when installing a secondary tide station. Without 

 sacrificing anything which might impair the reliability of the record, 

 the installation of a tide station to be occupied for only a short period 

 of time may be somewhat simplified. 



179. The tide staff may be a plain board with graduated scale 

 nailed to a convenient support. The staff must, however, be con- 

 nected with a system of bench marks as described in paragraphs 97- 

 112. The smaller and more conveniently installed portable auto- 

 matic tide gage may usually be substituted for the larger standard 

 gage. The float well may be a simple construction of plain boards; 

 or, if the portable automatic gage is used, made up of convenient 

 lengths of standard 4-inch iron pipe. If the observations are taken 

 during the summer time only, no tide house is necessary, a waterproof 

 box affording sufficient shelter for the standard gage, and the iron 

 cover provided with the portable gage serving as ample protection 

 for the latter. In the winter time a tide house is more or less necessary 



