196 On the Arcs perpendicular to the Meridian, 



of the French government, and by Mr. Herschel and Captain 

 Sabine, on that of the British. The arrangement of the interme- 

 diate stations was the same as that which we are now considering ; 

 viz. three signal and two chronometdcal stations. The execu- 

 tion took place at the same period as the unsuccessful attempt 

 of that year, between Paris and Brest, one of the signal stations 

 (that nearest to Paris) being common to both arcs. In this 

 case also, four nights only of the twelve succeeded, although, 

 from the accordance of the partial results obtained on those 

 nights, the general conclusion is considered as satisfactory, and 

 as not requiring repetition. 



It may not be without its use to collect, in one view, what 

 experience has shown as to the proportion of powder proper to 

 be employed in the signals ; because, whilst on the one hand 

 the quantity must be sufficient to insure the visibility of the sig- 

 nals, it is desirable also on the other to be guarded against 

 excess, since the smaller the quantity of powder the more instan- 

 taneous is the inflammation. In the instructions given by the 

 6t^t-major-general, at Vienna (Ephem. di Milano, 1826, Ap- 

 pendix, p. 46,) to the Officers employed to make the signals on 

 Monte Baldo, which were to be seen at Milan, a distance of 

 seventy-two geographic miles, the quantity of powder directed 

 to be used was f of a Viennese pound, to be increased to a pound 

 in unfavourable states of the atmosphere. The signals were seen 

 at Milan, by M. Carlini, who makes no particular comment con- 

 cerning them ; but in the report of Professor Santini {idem. 

 Appendix, p. 67,) by whom they were also observed at Padua, 

 forty- three geographic miles from Monte Baldo, after noticing 

 that the signals were so distinctly visible as to be observed by the 

 naked eye, as well as with glasses, he remarks, that they could 

 not be said to be absolutely instantaneous ; that the progress of 

 the inflammation was perceptible, and that the illumination 

 lasted, from its first appearance to its total extinction, about a 

 quarter of a second, or, perhaps, even one-third of a second ; 

 which obliged him to endeavour, as far as was possible, to ap- 

 preciate and register the instant of the greatest splendour. We 

 conclude, therefore, that, for distances of from forty to fifty 

 miles at least, the quantity directed by the authorities at Vienna 

 was excessive ; and this is further confirmed by the fact, that 



