122 



escape, and the arm falls down again to its proper height until the 

 pendulum carries it off, the pressure of the long teeth on the stops 

 not being enough to hold it up. For these reasons also there is no 

 difficulty in satisfying the mathematical condition investigated by 

 Mr. Denison in a paper read before the Society in 1848, viz. that y 

 (the angle at which the pendulum leaves one arm and takes up the 



other) should =— ,— , or at any rate not be less than ■;- (a being the 



extreme arc of vibration). The escapement requires no oil in the 

 parts affecting the pendulum ; and it contains no delicate work, and 

 is very easy to make ; and as a highly finished train will be no 

 longer necessary, astronomical clocks may be made on this plan much 

 cheaper, as well as better, than heretofore. 



In turret clocks an escapement of this kind supersedes the 

 necessity for a remontoire in the train to equalize the force on the 

 scape-wheel, and also of long and heavy pendulums, which are expen- 

 sive when compensated, and are sometimes difficult to fix. It will 

 also allow cast-iron wheels to be used throughout the clock (which 

 Mr. Dent has now used for several years in connexion with Mr. 

 Denison's spring remontoire for the train), as the friction of the train 

 can no longer affect the pendulum. 



February 21, 1853. 



Professor Challis gave a Lecture on Halos, Parhelia, and Para- 

 selense. 



March 7, 1853. 

 Professor Stokes gave an account of some further researches 

 relating to the Change of Refrangibility of Light. 



April 11, 1853. 



The Rev. Mr. Pritchard, F.R.S., gave an account of the Processes 

 requisite to render Quicksilver tremorless for Astronomical Obser- 

 vation. 



The great improvements recently introduced, and especially by 

 the present Astronomer Royal, in the construction and methods of 

 using astronomical instruments, require a far more extended use of 

 reflexion from mercury than heretofore. Unfortunately, however, 

 both the convenience and the accuracy of these methods have been 

 greatly limited and impaired by the tremors to which mercury is 

 liable. Many attempts have been made both in France and in Ger- 

 many to remove or obviate these tremors, but hitherto by no means 

 with perfect success. The Rev. C. Pritchard, of Clapham near 

 London, has proposed a method which appears fully adequate to the 

 requirements of astronomy. It consists in the adoption of a silver- 

 plated or amalgamated copper vessel of a peculiar form, admitting 

 the use of a very thin stratum of mercury without the necessity of 

 an inconvenient amount of shallowness in the vessel itself. Mercury, 



