OBSERVATIONS IN PHYSICAL OCEANOGRAPHY 37 



ing instruments to great depths, and it must have laboratory space for 

 such examinations of water samples as need to be made shortly after 

 collection. The arrangements on board ship vary widely, however, 

 depending upon the design of the vessel. 



Because of the manner in which oceanographic data have to be 

 obtained, studies in ''synoptic oceanography" are in general prohibitive 

 as to cost, and simultaneous observations have not been made except in a 

 few instances when vessels have cooperated within a very limited area. 

 Fortunately, the oceans are much closer to a steady state than is the 

 atmosphere, and therefore observations that have been made by the same 

 ship within a reasonably short time can be treated as if they were simul- 

 taneous (p. 106). The now classical example is the work of the Inter- 

 national Ice Patrol, which is conducted by the U. S. Coast Guard. 

 During the spring months a vessel of the Ice Patrol makes cruises at 

 intervals of about four weeks, each cruise lasting about two weeks and 

 covering the region to the east and southeast of the Grand Banks of 

 Newfoundland. At the end of each cruise the currents are computed 

 (p. 106), and on the basis of these computations the drift of icebergs is 

 predicted. 



Temperature Observations 



Three types of temperature-measuring devices are used in oceano- 

 graphic work: (1) accurate thermometers of standard type are employed 

 for measuring the surface temperature when a sample of the surface 

 water is taken with a bucket, (2) reversing thermometers are used for 

 measuring temperatures at subsurface levels, and (3) thermographs are 

 employed at shore stations and on board vessels for recording the temper- 

 ature at some fixed level at or near the sea surface. The centigrade 

 scale is the standard for the scientific investigation of the sea. A high 

 degree of accuracy is necessary in temperature measurements because of 

 the relatively large effects that temperature has upon the density and 

 other physical properties and because of the extremely small variations 

 in temperature found at great depths. Subsurface temperatures must 

 be accurate to within less than 0.05°C, and under certain circumstances 

 to within 0.01°C. Such accuracy can be obtained only with well-made 

 thermometers which have been carefully calibrated and rechecked from 

 time to time. Because of the greater variability of conditions in the 

 surface layers, the standards of accuracy there need not be quite so high. 



Surface Thermometers. Conventional-type thermometers used 

 for surface temperatures must have an open scale that is easy to read and 

 that is provided with divisions for every 0.1°. The scale should prefer- 

 ably be etched upon the glass of the capillary. The thermometer should 

 be of small thermal capacity in order to attain equilibrium rapidly, it 

 should be checked for calibration errors at a number of points on the 



