1 88 LIGHT SCIENCE FOR LEISURE HOURS. 



the private observatory of Sir T. M. Brisbane at Para- 

 matta. In 1825, the comet was detected by several 

 independent observers. It was seen again in 1828, 

 being detected by two observers Harding at Gottin- 

 gen, and Gambart at Marseilles. In 1832 and 1835, 

 it was seen from the observatory at the Cape of (rood 

 Hope. 



At the next return of the comet, which took place 

 on December 9, it was visible to the naked eye for the 

 first time since its discovery. At this passage, also, a 

 very noteworthy peculiarity was remarked or rather 

 a peculiarity which had been remarked by Encke in 

 1818, was now, for the first time, placed beyond a 

 doubt. Encke had suspected that the comet's period 

 was slowly diminishing. Each return to perihelion oc- 

 curred about two and a half hours before the calculated 

 time. Such a discrepancy may appear very trifling, 

 and in fact it might seem that no certainty could be 

 felt respecting it ; and this is the case so far as one or 

 two revolutions are concerned. But when each succes- 

 sive revolution shows the same discrepancy, the defi- 

 ciency soon mounts up to a period respecting which 

 no doubt can be entertained. For example, between 

 the perihelion passage in 1789 and that of 1865, the 

 comet has made twenty-three revolutions, and each 

 has been less than the preceding by two and a half 

 hours (on the average). Hence, the last revolution of 

 the series occupied two days and a half less than the 

 first. But even this does not express the full effect of 

 the change ; for the comet having gained two and a 



