THE TERRA NOVA GOES SOUTH 57 



There had been little so far which came into the province 

 of geology, but from this time forward the three geologists 

 (Priestley, Debenham, and Taylor), and the physicist (Wright) 

 formed an " Iceberg Watch." Day and night since the 9th 

 every berg in sight has been noted and catalogued as tabular, 

 domed, tilted, or pinnacled. All within three miles have been 

 sketched and many photographed. Their distance has been 

 determined by the range-finder, and their height by the 

 sextant. 



The range-finder is a tube four feet long, containing a 

 prism at each end and an eyepiece in the centre. The instru- 

 ment is mounted on a heavy rotating standard, and the 

 observer looks into the side of the instrument (as it were 

 across the middle), and not lengthwise as in a telescope. 

 Through one prism appears the image of the upper half of 

 the berg, through the other prism (which can be rotated on 

 a vertical axis) the image of the lower half of the berg. 

 Obviously, if the object is very far away, the rays of light 

 constituting these two images are nearly parallel. If the berg 

 is nearer, the movable prism must be twisted inwards to make 

 its image fall correctly under that of the fixed prism. (From 

 the end prisms it is a simple matter to deflect the images again 

 into the same central eyepiece.) The amount of rotation of 

 the right-hand prism measures the distance of the object. 



A somewhat similar optical arrangement is made use of in 

 the sextant. Here, however, a mirror image of one object 

 is made to coincide, by moving an arm of the sextant, with 

 the direct image of another object. The angle between the 

 two objects — say the top and bottom of a big berg — is thus 

 obtained. We have found the distance by the range-finder, 

 and by a simple calculation can get the height in feet. The 

 sextant will also give the angular width of the berg, and as 

 we know the distance, as before we can find the width in feet. 



Within a few hours of the first icebergs we reached the 

 pack-ice. At first a few solitary spongy pieces of ice only 

 a foot or two across, and so tumbled and broken by the waves 

 that we were doubtful if they were not fragments of one of 

 the bergs in the offing, rather than outliers of the true pack. 

 But by noon we were cutting through it, and from that time 

 it got thicker and more formidable as we penetrated south- 

 ward. In this region (65 S.) it lay in long streaks across 



