F. GEOGRAPHY. 231 



RECENT EXPLORATIONS IN AUSTRALIA. 



We find in Ocean Highioays a sketch of certain explora- 

 tions in Australia in 1872, organized by Baron F. von Mul- 

 ler, of Melbourne, and commanded by Mr. Ernest Giles. The 

 result of their labors has been to fill in much topographical 

 detail between 129 and 134 east longitude and 23 and 25 

 south latitude. 6 A^ June, 1873, 132. 



ON ASCERTAINING A SHIP's PLACE AT SEA. 



Professor Rogers, of Harvard College Observatory, has un- 

 dertaken the interesting problem of determining accurately 

 the average number of miles that a ship may be out of her 

 reckoning. He states that, in the case of British vessels, 

 there is a continual increase in the proportion of wrecks ; 

 thus those in 1858 were thirty-eight per cent, more than in 

 1848; in 1868, they were forty-four per cent, more than in 

 1858. For 1869, we have a decrease in the number of vessels 

 of four per cent., and an increase in the number of wrecks of 

 twenty-one per cent. The confidence in reckoning by astro- 

 nomical observations had increased the danger from this 

 source. Professor Rogers says that wrecks are caused 



1. By causes beyond control. 



2. In order to obtain insurance. 



3. From unknown deviations of the magnetic compass. 



4. By errors of astronomical observation. 



He concludes that seventy per cent, of wrecks might have 

 been prevented by due precautions. The number of insured 

 vessels that are wrecked is 3.3 times as great as the number 

 of uninsured vessels. He shows that from the errors in the 

 rates of the chronometers, an error of position of 3.6 miles 

 must be expected ; that an error of 11.5 miles is not improb- 

 able, and that an error of 21 miles may occasionally occur. 

 All of this computation assumes that the navigator has an 

 average chronometer. In these instruments, as is well known, 

 the most important source of error is found in the varying 

 temperature. Professor Rogers concludes that the navigator 

 who assumes that he can get the place of a ship certainly 

 within five miles, or, probably, within fifteen, exhibits an over 

 confidence which may lead to his ruin. In this connection 

 it is well to recall the great labors of Mr. Hartnup, of Liver- 



