' ' MARIOISr ' ' EXPEDITIOISJ' TO DAVIS STRAIT AND BAFFIN BAY 57 



Had there been no cloudy weather nor unusual refraction, the 

 positions at sea would have been very good, as such positions go. 

 Unfortunately, there were frequent periods when bad weather made 

 the heavenly bodies invisible and a few other periods when abnor- 

 mal refraction was suspected. The accuracy of the track positions 

 in some places at sea is, therefore, doubtful, though, in view of the 

 paucity of previous soundings, almost never to a seriously damaging 

 degree. Throughout all periods when sights w^ere possible, many of 

 them would be taken, permitting good estimates and interpolations 

 to be made for the dead-reckoning positions which had to be relied 

 on between the fixes — that is, during the bad weather periods. 



The ship's track was always determined graphically by laying 

 down and running forward the various bearing and Sumner lines 

 on large-scale plotting charts that w^ere on the Mercator projection. 

 The regular plotting sheets published by the Hydrographic Office of 

 the United States Navy were used south of latitude 61° N. Special 

 charts constructed on board on a scale of 2.75 inches to the degree 

 of longitude were used for tracking farther north, as no regular 

 plotting sheets could be purchased i^rior to the departure of the 

 expedition for latitudes above the sixty-first parallel. 



The observations were worked out principally by means of Hydro- 

 graphic Office Publications 203 and 204 when south of the latitude 

 60° 30' N., and by means of the haversine-cosine formula given in the 

 American Practical Navigator when farther north. Meridian and 

 ex-meridian sights were worked out during the entire cruise by 

 means of the methods given in the latter publication. 



At least 10 sun sights were worked out on each clear to partly 

 cloudy day. On days that were mostly overcast almost as many sun 

 sights would usually be worked out, for, due to the desire to keep 

 track of the position accurately, the intervals between observations 

 were regularly shortened whenever the sun appeared to be in danger 

 of being obscured for a long period by clouds. 



The positions were carefully determined and checked and are be- 

 lieved to be free from any gross errors. Considering the number of 

 sights and bearings taken and the methods of navigation used, they 

 are probably the best results that could have been obtained. 



In general, the positions of the soundings taken off the coast can 

 be considered as reliable to fairly reliable, possible exceptions being 

 off Baffin and Resolution Islands, where unusual refraction may have 

 prevailed. On the Marion we seldom had positive evidence of un- 

 usual refraction, such as great distortion of objects near the horizon 

 and failure of sights taken at close intervals to agree well with one 

 another. It is thought that the references in the sailing directions 

 about the unusual refraction to be expected in the Davis Strait region 

 can be attributed in many cases to the land's being incorrectly plotted 

 on the existing charts. 



CORRECTIONS APPLIED TO THE ECHO SOUNDINGS 



It is well known that echo soundings are not accurate as taken. 



They have to be corrected for instrumental and personal errors ; also 



for errors arising from the varying speed of sound in the water 



column under the ship. The fathometer on the Marion was con- 



68165—32 5 



