358 DISCOVERY REPORTS 



dives are made which are neither so long nor so deep as the sounding dives. A number 

 of these shallow surface dives always intervenes between two "sounds". Although 

 the usual duration of a " sound " is between ten and twenty minutes, conversations with 

 Norwegian gunners have revealed instances of longer dives than this. The Greenland 

 Right Whale is said to be able to make sounding dives lasting up to forty-five minutes. 

 As to the depths to which whales may descend when sounding very little indeed is 

 known. Scoresby, jun. (1820, p. 468), stated that the Greenland Right Whale has been 

 known to reach eight hundred fathoms. Such a depth, however, is extremely im- 

 probable. Racovitza (1903, pp. 17-19) considers that, when all factors have been taken 

 into account, 100 metres is the maximum depth attainable. It is probable that the 

 depths which whales can reach have been greatly overestimated in the past and that 

 observers have taken the long periods of submergence to imply great depth. The 

 limiting factor in any dive is the danger of caisson disease (liberation of nitrogen bubbles 

 in the blood and tissues, especially the nervous tissues) upon returning suddenly from 

 depths where the excess hydrostatic pressure is greater than 1-25 atmospheres (2-25 

 atmospheres absolute pressure). Human divers can descend to a depth where this 

 pressure obtains (about 42 feet or 7 fathoms) and return to the surface with impunity ; 

 but at greater depths, involving greater pressures than this, precautions for the pre- 

 vention of caisson disease must be taken, depending on the duration of the exposure 

 to the increased pressure (Boycott, Damant and Haldane, 1908, p. 359). At a depth 

 of 60-66 ft. (3 atmospheres pressure) a human diver can remain for a quarter of an hour, 

 provided that his ascent to the surface occupies two minutes. He can remain at 96- 

 108 ft. for a similar period, provided he takes in all 11 minutes to reach the surface 

 (with a stop of 3 minutes at 20 ft. and 5 minutes at 10 ft.). At this depth the absolute 

 pressure is 4 atmospheres. There is no a priori reason to suppose that a whale is less 

 susceptible to caisson disease than any other mammal, but we may believe that it has 

 become accustomed to slightly higher pressures, longer exposure, and to returning from 

 these depths rather more quickly than a human diver. Even so it seems improbable that 

 any organism could withstand the effects of swift release from pressures greater than 

 5 atmospheres — corresponding to a depth of 132 ft. 



It may be said, then, that a whale probably does not descend to depths much greater 

 than 130 feet, but can remain below for periods of up to half an hour. In considering 

 the effects of diving upon the physiology of the whale, the purely mechanical action of 

 hydrostatic pressure can probably be discounted as a factor. In any case the purely 

 mechanical effects of pressure during a dive are very slight. Animals under observation 

 have been submitted to pressures of nearly 40 atmospheres and have apparently suffered 

 no inconvenience (Report to the Admiralty of a Committee on deep-water diving, p. 1 1), 

 while men in caissons can work freely at depths of 15 fathoms (about 4 atmospheres 

 pressure) provided that ventilation is adequate. The pressure, however, during the dive 

 will have a certain effect in increasing the partial pressure of the oxygen in the lungs. 

 Paul Bert found that for a series of animals and birds oxygen became toxic at a partial 

 pressure of 4 atmospheres. This partial pressure, however, corresponds to 15-20 



