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WHALES 



Figure 75. Humpback inhaling. Note that the blowhole is wide open. 

 Photograph : W. H. Dawbin, Sydney. 



whenever a gas escapes under pressure, it becomes cooled. We all remem- 

 ber from our elementary science lessons that when a gas expands it loses 

 heat, and from the way the whale whistled when it blew, we could tell 

 that the air was being forced through a narrow opening under great 

 pressure, subsequently to undergo great expansion outside. It is here that 

 the moisture (which is present in all exhalations) condenses by cooling 

 and turns into the small visible drops that constitute the blow. This 

 explanation is confirmed by an observation of Gilmore (i960): when 

 California Grey Whales exhale very slowly, the blow is hardly visible. 

 Recently, F. C. Fraser and P. E. Purves, both of the Natural History 

 Museum in London, offered a new explanation of the blow. In the trachea 

 of all Cetaceans there is a foamy substance, which is also present in the 

 bronchi of rabbits. Moreover, the wall of the auditory air sacs (see 

 Chapter 7) produces foam in its large glands. The authors claim that 

 the foam has a strong affinity for nitrogen. We shall return to this question 

 later, and meanwhile note that Fraser and Purves believe that during 

 every blow large quantities of this substance are exhaled and that it is 

 droplets of foamy mucus which we see as the blow. However, as long as 



