1824.] Mr. Baily on the ensuing Opposition of Mars. 107 



colouring ingredients, which I consider to be the oxide of cop- 

 per and the oxide of silver, would be useless. The colouring 

 matter which forms only a film of at most l-200th of an inch in 

 thickness upon a substance of glass varying from l-30th to l-10th 

 of an inch, is quite sufficient reason for desisting. I attempted some 

 time since to grind the uncoloured portion away ; and in another 

 instance, to detach it by fluoric acid, but in each of these 

 attempts I was unsuccessful. That class of your readers to 

 whom this communication may be of any service, if not fully 

 aware of the proportions of the colouring oxides, may easily 

 obtain them by a few experiments. 



Article VI. 



On the ensuing Opposition of Mars. By F. Baily, Esq. FRS. 

 VP. Ast. !Soc. (Read before the Astronomical Society of 

 London, Jan. 9, 1824.)* 



At a time when we have two new and excellent observatories 

 established in the southern hemisphere, where the celestial phe- 

 nomena are watched and observed with the greatest diligence 

 and zeal, it becomes the more important and necessary that 

 corresponding observations of a certain class of those pheno- 

 mena, of not very frequent occurrence, should also be made in 

 the northern hemisphere, by such persons as are fortunately pos- 

 sessed of the requisite means for this purpose. Without this 

 co-operation, -the labours of those industrious observers will lose 

 much of their value, and the advantageous opportunity of eluci- 

 dating an important branch of physical astronomy will be 

 wholly lost to the public. 



The ensuing opposition of Mars, on the 24th of March, is one 

 of this class : a phenomenon which occurs once only in a period 

 of about 780 days. It is well known that corresponding obser- 

 vations of this planet, in the two hemispheres, as compared with 

 stars situated near its path, about the period of its opposition, 

 will serve to determine its parallax. And the parallax of Mars 

 being known, that of the sun may thence be deduced. This was 

 the plan adopted by Lacaille, when he was at the Cape of Good 

 Hope, in the year 1751 ; since which period, the method has 

 fallen into disuse for want of an observatory in the southern 

 hemisphere, with instruments fit to be compared with those in 

 Europe. 



The present period seems extremely favourable (for the reasons 

 above mentioned) for the revival of this method. At the time of 



• See our report of the proceedings of this Society in the present Number. — EdU. 



