MANUAL OF TIDE OBSERVATIONS 81 



tide varies height corrections will be necessary. For the adjustment 

 of tide reducers between stations along the coast and inside waters 

 it will be found convenient to divide the area covered into sections. 

 Each section may cover an area in which the variation in range of 

 tide does not exceed three-tenths of the unit used for the tide reducers; 

 that is, 0.3 foot, 0.9 foot, and 1.8 feet when the reducers are entered in 

 units of a foot, half fathom, and whole fathom, respectively. Take, 

 for example, an area 15 miles long with no narrow restrictions and 

 with depths of 3 fathoms or less. At one end is station A, where tide 

 observations have been taken during the time of the soundings; at 

 the other end is station B. where the time differences and ratio of 

 ranges have been determined. The mean range of tide at station A 

 is 2.6 feet, at station B 3.5 feet, the difference in range being 0.9 foot. 

 The difference in the time of tide between the two stations will be 

 assumed to be 45 minutes. The area should, therefore, be divided 

 into four sections. Assuming the tide to increase uniformly with the 

 distance, the first section will be 2i/2 miles long and the height and 

 time of the tide the same as at station A. The second section will 

 be 5 miles long and the height of high water 0.3 foot greater and time 

 15 minutes later than at station A. The third section will also be 

 5 miles long and the height of high water will be 0.6 foot greater and 

 the time 30 minutes later than at station A. The fourth section will 

 be 2^ miles long and the height of high water 0.9 foot greater and 

 the time 45 minutes later than at station A. 



258. The tide reducers for soundings in each of sections 2, 3, and 4 

 may be derived directly from the curves for station A by reading 

 the curves at points which are as many minutes earlier than the times 

 of the soundings as there are minutes in the time allowance for each 

 section and multiplying the readings by the ratio of ranges. For 

 offshore areas the range of tide may generally be taken the same as at 

 -the nearest point along the coast. 



TEMPERATURE AND DENSITY OBERVATIONS 



259. At some tide stations, temperature and density observations 

 are taken incidentall}- to the main purpose of securing the tide record. 

 These observations are taken daily at the time the tide station is 

 visited and are recorded in form 457 (fig. 29). The water to be 

 tested must be dipped from just below the surface. The order of 

 procedure in making the observations is indicated in detail on the 

 back of the form. The record is to be forwarded to the office at the 

 close of each calendar month. 



TEMPERATURE 



260. In general, the temperatures will be given to the nearest tenth 

 or nearest half degree in the centigrade scale. The scale of the 

 centigrade thermometer is usually divided into degrees and half 

 degrees. If a Fahrenheit thermometer is being used, it should be 

 indicated by the letter F at the top of the column containing the 

 temperatures. The record includes the temperature of the outdoor 

 air, the temperature of the sea water immediately after it has been 



