124 The Rev. Dr. Robinson on the Difference of Longitude 



have been owing to the bad quality of the rockets employed, as they are said to 

 be similar to those furnished for the English operation, which proved defective, 

 a large proportion of them bursting. They were, in fact, overloaded, the sig- 

 nals being given with eight ounces of powder ; and it seems that in attempting 

 to make them able to carry this to the requisite elevation, the limit of strength 

 was approached rather too closely. None of the distances are excessive. That 

 (La Heve, St. Clair) which in the first line barred all transmission, is but 

 seventy-one statute miles ; it however required an elevation of 680 yards, which 

 probably many of the rockets did not reach. Colonel Bonne, who reports this, 

 attributes the failure to the fog which rests on the Seine, as the line of sight 

 crossed this river seven times ; and seems to think that in all such operations, 

 the passing large surfaces of water should be avoided. Before adopting this 

 conclusion, we should remember that in 1825, when the line was changed, and 

 when no distance exceeded fifty-two miles, no greater success was obtained. 

 Perhaps sufficient attention was not paid to the selection of clear nights for the 

 signals ; as every astronomer is aware that sometimes small stars can be seen 

 almost to the horizon, while in ordinary good observing weather, this is by no 

 means the case. When such favourable circumstances are noticed at the 

 observatories, which are the extremities of the chain, a transmission of signals by 

 numerous intermediate posts, should run along the line as a notice to fire the 

 rockets, and thus success may be insured by a moderate expenditure of blue 

 lights and patience. 



The operations on the arc between Greenwich and Paris are described by 

 Sir John Herschel in the Philosophical Transactions for 1826, with his usual 

 precision and elegance : the memoir explains the method of successive signals 

 with peculiar clearness, and in particular illustrates the method of using the 

 broken sets to the best advantage. The distances here also were moderate, the 

 greatest (La Canche, Lignleres) being only fifty-six miles ; yet the success was 

 not very great, ten complete transmissions being obtained only on four nights out 

 of twelve, by 120 signals at each of the three stations. It Is however evident, 

 that Colonel Bonne's opinion of the difficulty of passing water does not hold 

 with respect to sea ; for, while 109 of the Wrotham signals were seen at 26 

 miles, ninety-two of those at La Canche, at fifty-two miles, were visible. 



These operations were not followed up in Great Britain for several years. 



