six o'clock
`` We are out for the purpose of testing the boat . To make an attempt to round the cliff and steer it into the mouth of the river in this wind will be the best test of its maneuvering ability . '' As stated , the wind was now blowing from the southwest , and they were northeast of the mouth of the stream they wished to enter . They stood out to sea in order to make a starboard tack , and it was a gratification to see the magnificent manner in which the vessel responded , and before six o'clock they found themselves sailing up the river , and safely landed at the boathouse . An examination showed that the crossbeam supporting the mainmast was split from end to end , and only the roof structure held it in place . Thus the trip had a warning lesson for them , and Harry was not slow to take advantage of it and install a larger crosspiece . George had entirely forgotten the incident of the calcareous slab which had on it the tracings of the cave , and which had been the means of giving the Professor the first hint that they were in a pirate 's cave .