242 
POPULAR SCIENCE REVIEW. 
pole of the planet lies. I do not here enter into the considera- 
tion of the method of calculating from the observed angle of 
position the true position of the axis, simply because the reader 
lor whom the explanation would have any meaning will be able 
quite readily to obtain the required formulae for himself. They 
are presented in an article of mine upon the opposition of Mars 
in 1873 in volume xxxiii. of the 66 Monthly Notices” of the 
Astronomical Society. But as there are some who find formulae 
perplexing, especially when the manner in which such formulae 
have been obtained is not indicated, I deem it desirable to in- 
dicate a construction by which the proper bearing and position 
Tv 
of Mars at any time can be easily and exactly indicated. It 
can easily be shown that this construction corresponds with the^ 
spherical formulae given for such cases in the above-mentioned 
volume of the “ Monthly Notices,” and also in the explanation 
of Tables VII. and VIII. of my work entitled “ Saturn and its 
System.” 
Describe a circle apb, and let pop', aob be diameters at 
right angles to each other. From p measure on pb an arc pi> 
■ 
