359 



I shall only add one general remark, which is, that if we lengthen the time 

 of the rotation but 2 minutes, it will throw the last observation back above 1 16" 

 and if we diminish it by 1 minutes, there will arise an excess of more than 117 

 and in either case the calculations and observations would be totally at variance 

 from which we may conclude that our period must be exact to much less than 2 

 minutes either way. Indeed, what alterations may have taken place in the belts 

 themselves, it is impossible to determine. That there have been some, we may 

 admit, but may suppose we have no particular reason to suspect them to have 

 been very considerable. And, after we have shown that a proper motion, in the 

 spots of the belts, of 1 16° one way, or of 117 the other, would only occasion 

 an error of 2 minutes in time, we need not hesitate to fix the rotation of the 

 planet Saturn on its axis at lO" l6™ 0^4. 



IX. A Method of Measuring the Comparative Intensities of the Light emitted 

 by Luminous Bodies. By Sir B. Thompson, Count of Rmnfvrd, F.R.S. p. Qy . 



The method is shortly this; Let the 2 burning candles, lamps, or other lights 

 to be compared, a and b, be placed at equal heights on 2 light tables, or move- 



