342 HYDROGRAPHICAL SURVEYING [chap. xiv. 



meridian distance from A to B, dependent upon A and K being 

 in certain longitudes. 



We can use the same means if we call at more places than 

 one on the way between A and K, but each stoppage will 

 probably detract horn the value of the sea rate. 



We are here using the sea rate only, and therefore shall take 

 the date of the last observations at departure and first on 

 arrival as the epochs for calculation. If we have obtained 

 rate on departure and arrival, we shall gain valuable informa- 

 tion about our chronometers, as we shall be able to see how far 

 they have obeyed any theory as to gradual or uniform change of 

 rate, according to the ordinary assumption that the sea rate 

 is the mean of the two harbour rates. 



The value of a meridian distance by this method will, as 

 always, be influenced by the conditions of temperature, fair 

 passage, etc., which must therefore be taken into consideration 

 and recorded. 



It will be remarked that by this methoi a large amount of 

 time is saved, and opportunities otherwise wasted are utilised 

 to their full extent. Instead of the necessity of waiting, 

 certainly at A and K, and perhaps at B as well, for from five 

 to eight days, a simple call of a few hours at each is sufficient 

 to obtain an excellent result. Moreover, instead of involving 

 the eccentricities of chronometers during the time in harbour 

 at each end, we only include in the calculation the actual time 

 while travelling at sea, and thereby save the irregularities of a 

 good many extra days. 

 Shad- Captain Shadwell, in treating of this case, does not use an 

 f^f^^ invariable sea rate pure and simple, but supposes that the rate 

 ment. of departure has gradually and uniformly changed into the 

 sea rate, which he considers as the rate on the middle day of 

 the passage only. He therefore applies for his determination 

 of B from A an intermediate rate between the sea rate and 

 rate of departure ; but our experience does not lead us to think 

 that tliis is an advantage, although by doing the same to 

 th3 sea rate and rate of arrival, he gets a second meridian 

 distance from B to K, and takes the mean of the two as 

 his result. Our opinion is that, temperature being left out 

 of the question, a better result is likely by using a uniform 

 sea rate. 



