l88 CHALLENGER 



beneath the buoys it can be picked up through the water by the 

 hydrophone and again a signal is sent to the ship. The depth of 

 water both beneath the buoys and beneath the ship has been found 

 with the sounding machine, and the time of the passage of sound 

 from the ship to the sea-bed and from the sea-bed to the buoy 

 hydrophone can be calculated. This time is subtracted from the 

 total time for passage of sound, the resulting time being that taken 

 by the sound to pass horizontally through the sediment layer for 

 a distance of eight miles, so that the speed of sound in the sedi- 

 ment layer has been discovered. 



Yet other paths of sound pass through the sediment layer to 

 the harder, more compressed layers beneath, in which sound 

 travels more quickly. The time of the passage of sound through 

 these lower layers is also timed, and a speed of sound for these 

 layers found. When the speed of sound in a material is known 

 then geophysicists get a pretty good idea of the type of rock 

 forming this material, relating it to their experiences on land 

 where borings can be made and samples of the material inspected 

 after measuring the speed of sound by seismic methods. 



By carrying out these seismic refraction experiments at a num- 

 ber of increasing ranges from the sono radio buoys, and knowing 

 the speed of sound in the material forming the layers, the vertical 

 thickness of the layers may be measured. 



Thus a picture is built up of the types of material which form 

 the layers of the earth's crust beneath the oceans, and such ex- 

 periments, carried out at a number of different positions in the 

 various oceans, can be seen to be complementary to the researches 

 of the Albatross regarding the sedimentary layers. 



In the winter of 1949-^0 Challenger lay in Chatham Dockyard, 

 enduring the usual discomforts of a refit and preparation for an- 

 other voyage. The crew were joining in twos and threes as they 

 were appointed from Portsmouth Barracks. They were living in 

 Chatham Naval Barracks and marching to the ship both morning 

 and afternoon, never a popular part of the commission, more 

 particularly on this occasion, as the men were from Portsmouth 

 Division, where Challenger^ s crew always came from : so that after 

 a day spent in the unheated store-rooms and in offices with scuttles 

 missing and rain dripping through the empty rivet holes, and after 



