20 



EIGHTH REPORT — 1838. 



sunrise ; and never was astonishment greater than mine at seeing it 

 riding high in the sky, broadly visible to the naked" eye, when pointed 

 out to me by a notice from Mr. Maclear, who saw it with no less 

 amazement on the 24-th. The next remarkable feature is the enor- 

 mously rapid rate of dilatation of the disc and the absorption into it of 

 all trace of the surrounding nebula. Another, is the interior cometic 

 nucleus. All these phenomena, while they contradict every other hy- 

 pothesis that has ever been advanced, so far as I can see, are quite in 

 accordance with a theory on the subject which I suggested on the oc- 

 casion of some obser\'ations of Biela's comet, — a theory which sets out 

 from the analogy of the precipitation of mists and dews from a state of 

 transparent vapour on the abstraction of heat. It appears to me that 

 the nucleus and grosser parts of the comet must have been entirely 

 evaporated during its perihelion, and reprecipitated during its recess 

 from the sun, as it came into a colder region ; and that the first mo- 

 ment of this precipitation was precisely that which I have pointed out 

 as the limit of the existence of the disc, viz. on the 21st of January, at 

 2h. P.M., or perhaps an hour or two later." 



On the Difference of Longitude between London and Edinburgh. By 

 Sir Thomas M. Brisbane, F.R.S. 

 Having observed the surprising accuracy with which the diiFerence 

 of longitudes of London and Paris had been obtained by Mr. Dent's 

 chronometers. Sir Thomas Brisbane applied to that gentleman, who, 

 with great liberality, furnished for the purpose of the experiments 

 twelve of his valuable chronometers. With these, the differences of 

 longitude of London, Edinburgh, and Mukerstoun were taken ; and by 

 a mean of all the obsei-vations taken in going to the latter station and 

 in returning, they were found to differ only by five one-hundredths of 

 a second. He exhibited to the Section the following table. 



