OCEANOGRAPHY ISELIN 255 



turbed. The permanent crew, exclusive of the scientific staff, 

 numbers 17 men. They have comfortable quarters, and there is 

 ample storage space below decks. The vessel is a smart sailer and 

 an excellent sea boat. Her rig, though small for the size of the hull, 

 is efficient and ideally adapted for heaving to, which is most 

 important for this type of work. 



The primary instrument of the physical oceanographer is the 

 deep-sea reversing thermometer. These are now sent down in 

 pairs, one open to the pressure of the water and the other inclosed 

 in a heavy glass case. The significance of this we will see presently. 

 Both the protected and the unprotected thermometers are of the 

 same construction. On being turned over the column of mercury 

 breaks off at a constriction in the capillary and the temperature is 

 read as the length of this detached thread of mercury. Thus the 

 reading can not be changed on the long haul to the surface by pass- 

 ing through the much warmer surface layers. The deep-sea ther- 

 mometers are now very accurate and can record the temperature to 

 a hundredth part of a degree centigrade. The pairs of thermometers 

 are sent down in frames which are mounted on the side of in- 

 struments known as water bottles. These are fastened to the wire 

 cable at suitable intervals and lowered over the ship's side. A small 

 v/eight, or messenger, is then slid down the wire, which, on striking 

 the uppermost instrument, closes the openings of the water bottle, 

 reverses the thermometers, and releases another messenger which 

 slides on down the wire and repeats the operation with the next 

 instrument. Thus a series of 10 or more water samples and pairs of 

 thermometer readings can be secured at one lowering. In deep 

 water it may take several such lowerings, each successively deeper, 

 to constitute a station. 



Although the thermometers and water bottles have been in use 

 for many years and have gradually become much improved in de- 

 sign, there was still one grave inaccuracy which has only recently 

 been eliminated. The depth of each observation was formerly re- 

 corded as the length of wire, measured on a wheel of known circum- 

 ference, from the sea surface to each water bottle. However, in gen- 

 eral the wire did not remain vertical in the water. The angle the 

 wire took depended on the relative motions of the ship, the surface 

 layers, and the deep, nearly motionless water masses. In other words, 

 the wire would not only enter the water at an angle, but probably 

 formed an S-shaped curve of unknown extent, so that the depths of 

 all observations was problematical. Since one of the purposes of 

 temperature and salinity observations is to assist in the study of 

 ocean currents — and in regions of currents the trouble of large wire 

 angles (often as much as 40° from the vertical) is at a maximum — it 



