4744 HYDROGRAPHIC MANUAL Page 410 



The sample bottles are placed in a frame around a central column containing a 

 pressure element similar to that in the bathythermograph. Each bottle has two stop- 

 cocks, one at top and bottom, which are operated simultaneously. In the upper part 

 of the pressure element are six slots, one opposite each bottle, which can be adjusted 

 in length to correspond to different depths. When the bottles are in place around the 

 central column the stopcocks are held open by triggers pressing against the upper part 

 of the pressure element. As the instrument is lowered through the water, the pressure 

 element contracts and each trigger in turn can enter its respective slot, releasing the 

 stopcocks which are closed by spring action. Thus, each water bottle in turn is closed 

 at a depth corresponding to the pressure for which the length of the slot is adjusted. 



4744. Bottom Specimen Cup 



The Belknap-Sigsbee specimen cylinder, commonly laiown as a detaching rod 

 (D in figs. 86 and 87), contains a device to obtain a sample of soft bottom and of the 

 water at the bottom, but its principal use is to release a sinker when the instrument 

 strikes the bottom. Only when the bottom is soft or oozy will a sample of the bottom 

 be obtained, and even in this case it may be mixed with the water. 



The water specimen cylinder has a poppet valve on its lower end which is held 

 open by the force of the water during descent but closes as soon as it is raised off the 

 bottom. During the descent water flows freely through the valve and through holes 

 in the top of the cylinder, both of which are closed on the ascent. The action of the 

 poppet valve is often ineffective as some of the bottom material frequently prevents 

 the valve from closing tightly and the water sample is lost, but in this case a sample 

 of the bottom material is usually obtained. The most effective instrument for obtain- 

 ing samples of bottom water, satisfactory for hydrographic purposes, is a Sigsbee 

 watercup attached to the sounding line a few fathoms above the bottom (see 4741). 



The sounding weights used with the Belknap-Sigsbee specimen cylinder are specially 

 cast pear-shaped weights with a vertical hole through which the cylinder passes to make 

 contact with the bottom. The weights are cast with heavy wire loops on opposite 

 sides by which they are suspended from a trigger at the top oi the instrument by a 

 seizing wire sling. This trigger is known as the Sigsbee releasing device and detaches 

 the weight when bottom is struck. (See also. 4665.) 



475. Hydrometer Sets 



The method usually used by the Coast and Geodetic Survey to determine the 

 salinity of sea water (see 633) is to measure the specific gravity of the water with a 

 hydrometer. Salinity and density are related by definition, so that if the relative 

 density or specific gravity is known, the salinity may be found by the use of tables or 

 a graph. By this method the salinity may be determined within one or two units in 

 the third significant figure, if the temperature of the water sample is measured to the 

 nearest 0.1° centigrade and the specific gravity to one in the fourth decimal place. The 

 temperature and specific gravity of a water specimen are measured with a hydrometer 

 set consisting of a hydrometer jar, a laboratory centigrade thermometer, and a set of 

 three hydrometers graduated for different ranges of specific gravity. These are illus- 

 trated in figure 89. 



There are other instruments used by scientists to measure directly the density of 

 sea water, from which salinity can be obtained; these are not ordinarily used by the 

 Coast and Geodetic Survey. The pycnometer gives very accurate results, but it 



