MAlSrUAL OF TIDE OBSERVATIONS 



83 



causing erroneous readings. When this occurs the broken sections 

 can usually be jarred together by hand. Readings of different ther- 

 mometers should be occasionally checked by comparison with 

 each other. 



261. The table below, which gives equivalent readings in centi- 

 grade and Fahrenheit thermometer scales, affords a convenient means 

 for converting the readings from one of these scales to those of the 

 other scale between the limits —20° and 39.5° C. Beyond these limits 

 the following formula may be used: Fahrenheit reading=32° + 

 (centigrade reading X 1.8) . 



Conversion of centigrade to Fahrenheit scale 



• When two values in the column for the centigrade scale are given for the same whole degree in the 

 Fahrenheit scale the one indicated by the asterisk should be taken when Converting the Fahrenheit readings 

 into the centigrade scale. 



DENSITY 



262. The unit of density is the density of fresh water at a tempera- 

 ture of 4° C. The actual density of the water at tide stations may 

 vary from a little less than unity for fresh water at a temperature of 

 more than 4° C. to approximately 1.031 for the heaviest sea water. 

 To provide for this entire range of density three hydrometers are 

 necessary — one with a scale ranging from 0.996 to 1.011, a second 

 with scale from 1.010 to 1.021, and a third with -scale from 1.020 to 

 1.031. The particular hydrometers to be used at any tide station 

 will, of course, depend upon the density of the water, and the observer 

 will select the hydrometers which will float with the stem partly 

 immersed. 



263. The hydrometers are numbered for identification and have 

 been tested by the Bureau of Standards. Tables of corrections will 

 be furnished when necessary. The number of the hydrometer used 

 for each observation should be entered in form 457. 



264. The method of reading the scale of the hydrometer, which is 

 graduated downward, is illustrated in figure 30. Except for densities 

 less than unity, the first two figures will always be 1.0, as printed in 

 the column of density in form 457. The two figures immediately 



