Page 417 equipment and instruments 478 



The casing has trunnions on the sides slightly above the center of gravity, to which a yoke is attached. "Where the velocity of the 

 current will not exceed 3 knots, the instrument is simply hung from a buoy by a nonferrous wire with a short length of brass chain to 

 permit it to orient itself freely. Where the velocity of the current exceeds 3 knots, in addition to Its suspension, the instrument is 

 connected horizontally to the anchor hne by a rigid leader to the trunnions. This leaves it free to swivel vertically on its trunnions 

 and horizontally about the anchor line. So constrained it will not sheer in any current. 



The streamlined buoy described in 2843 streams steadily without yawing in a 

 current, and when the current meter is suspended from such a buoy additional accuracy 

 of observations results. The instrument is suspended from the stern of the buoy and its 

 4-foot separation from the anchor yoke makes fouling extremely rare; and fouling is 

 eliminated entirely by the use of the leader connection to the anchor line. 



A radio current buoy can be placed at a station where current data are desired and 

 be left to broadcast its observations. The characteristic signals can be received by a 

 shore or ship radio receiving station anywhere within receptive range and recorded on a 

 chronograph. Or, for comparative purposes, a number of buoys, as many as 10 or 15, 

 can be operated simultaneously at different stations in an area. 



In addition to other personnel, operation of the buoys requires the services of a 

 qualified radio technician, and four chronograph observers trained to operate the radio 

 receivers and chronograph, to interpret and scale the chronograph tapes, and to record 

 the results. 



478. Other Oceanographic Instruments 



Ih addition to those instruments used or adaptable for use in hydrographic survey- 

 ing, there are other instruments used to collect data and specimens for oceanographic 

 and other studies. 



The sea water thermograph is an instrument designed to make a continuous auto- 

 matic record of surface water temperatures, which data are principally valuable in 

 meteorological studies. The instrument is made with either a bulb-and-capillary or 

 resistance-type temperature element which is inserted into the condenser intake pipe 

 or the keel of a vessel and is connected with a recording mechanism consisting of a 

 clock-driven drum on which a graph paper is placed. 



Ocean currents and the circulation of ocean waters are generally studied indirectly 

 by using computed densities, since masses of water moving along more or less well- 

 defined courses tend to retain then' physical and chemical properties. If the vertical 

 distribution of temperature and salinity is accurately known at two positions, compu- 

 tations may be made for the various depths from which the velocity and direction 

 of the current may be mapped. 



Ocean currents are also studied from direct measurements made with current 

 meters. There are numerous meters suited to various conditions, which record the 

 velocity by either an electric or a photographic mechanism and which utilize various 

 methods to obtain the directions of the current from a magnetic compass needle. 

 Direct cm-rent observations are made from vessels anchored at sea in great depths. 



Biological studies are made of plankton collected with instruments of three general 

 types. Silk townets for horizontal and vertical tows are generally used to collect speci- 

 mens. The closing type of townet is devised so that a messenger operates a releasing 

 device which closes the mouth of the net. The use of a closing net improves quantita- 

 tive determinations. Water bottles of large capacity, similar in construction to those 

 described in 474, are used to trap a considerable volume of water from which the plankton 

 catch is filtered out after the bottle has been hoisted to the surface. Definite quantita- 

 tive determinations result from their use, although the catch is very small. The 



