between the Observatories of Armagh and Dublin. 123 



Nor is such a quantity as ten pounds at all necessary in respect of visibility. 

 Von Zach found that even so little as four ounces was seen at 150 miles, by the 

 reflection of its light from the air, the flash itself being below the horizon ; and 

 that it was visible at 140 in the twilight:* and the French observersf state, 

 that at twenty-seven miles one-eighth of an ounce can be seen with the naked 

 eye. These are important as guiding facts ; at the same time, the superior 

 clearness of the air in the central parts of Germany should be kept in mind. 



This method was again forgotten till Von Zach revived it at the beginning 

 of this century. It has since been extensively used in Germany, J and by the 

 French and Italian astronomers in the measurement of an arc of longitude 

 between Marennes and Fiume.§ Where the localities of the line afford fit 

 stations, this method is very satisfactory ; but, where mountains of the requisite 

 height, and in proper places, are wanting, a sufficient elevation must be obtained 

 by art. I am not prepared to say how far it might be possible to obtain this by 

 " Captive balloons," though the fates of Pilatre de Rozier and Madame Blan- 

 chard are strong arguments against the union of aerostation and pyrotechny.|| 

 The use of rockets in such cases was proposed by Robins, in 1749» and was 

 practised by the elder WoUaston, and some other astronomers, near London, in 

 1775. More lately it was used on a large scale by the French, between Brest 

 and Strasbourg, and by a commission of French and English, between Greenwich 

 and Paris. The first is briefly described in the elegant notice by Major Sabine, 

 given in the Quarterly Journal, vol. xxili. ; and that part which was done in 

 1824 is given with sufficient detail in the Memorial du Depot de la Guerre, 

 vol. ill., to enable us to appreciate its value. It seems to have been unsuc- 

 cessful, as out of 300 signals, on each branch of the arc, only six transmissions In 

 the first attempt occurred on one branch, and none in the other ; and on the 

 second trial, out of 360, only thirty-six on the first. It is possible that this may 



* Correspondence Astronomique, vol. iii., p. 437. 



t Nicollet Con. des Terns, 1829, p. 381. 



X For details of some of these by Littrow, see Cor. Astron., vol. vii. p. 257. 



§ Con. des Terns, and Plana, Arc du Parallele Moyen. 



II Howitzer shells were tried by the French, but rejected, as the flash was not sufficiently bright ; 

 their fragments would, I think, be very dangerous to those who give the signals, and the howitzer 

 not easily managed on a mountain. 



r2 



