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



luminous object remains for one or two-tenths of a second on the eye : is this 

 duration the same for all persons ? Is there a corresponding delay in the per- 

 ception of light at its first appearance ; or, does the mind take instantaneous 

 cognizance of the action on the retina ? If not, is the interval of time required 

 the same for every observer ? The beautiful experiments of Mr. Wheatstone* 

 show that we can see an object whose visibility lasts only the millionth part of 

 a second ; but our perception of it may not be synchronous with its appearance. 

 All of this which concerns the astronomer might be decided by observing the 

 reappearance of the light, as well as its vanishing. The management of che- 

 mical apparatus on a mountain summit is, however, no easy matter, and Lieut. 

 Larcom, R. E., has suggested an application of the heliostat, which offers the 

 same results : directing its beam to one station, but diverting a portion to the 

 other by a second mirror, suitably placed, the same occultation and reappearance 

 may be effected with the utmost facility. The necessary apparatus was ready, 

 and if there had been enough of sunshine in May, I should have reported on the 

 performance of it ; but I hope that before these longitude operations are com- 

 pleted, I shall have another opportunity. 



No more mention of fire signals occurs in the annals of astronomy till 1735, 

 when De La Condamine proposed to measure an arc of longitude by means of 

 the flash of cannon ; taking the idea, in all probability, from the ridiculous pro- 

 ject of Whiston. As the signals are generally given on mountains, where 

 cannon are of difficult conveyance, his proposal is scarcely less absurd ; but it 

 was made practicable four years after by Cassini and Lacaille, who used the 

 powder without the artillery. Stationed on mountains, in the south of France, 

 110 miles apart, these astronomers observed the flash often pounds of powder 

 fired at an intermediate point, and deduced, though but imperfectly, the differ- 

 ence of longitude. Besides the imperfection of their means of getting the time, 

 the quantity of powder used was excessive, and its flame must have lasted one or 

 two seconds. Even with so small a quantity as half a pound, this inconvenience 

 is felt : Professor Santini complains that the signals given with this quantity, at 

 Monte Baldo, in 1824, were not instantaneous, the inflammation lasting ^ of a 

 second. It must, however, be observed, that this is more remarkable when the 

 powder is unconfined, than when fired in ordnance, or in the head of a rocket. 



* Philosophical Transactions, 1834, p. 591. 



