1877.] On the Beds of Chert in the Limestones of Ireland. 163 



which is sufficiently constant, our most successful results having been 

 obtained with a six-horse power Corliss engine, with a heavy fly-wheel, in 

 the workshop of the College. 



Some experiments which promise well have been made with rotating 

 disks on the principle of the thaumatrope. By using a rotating disk 

 with slits, and viewing through them another disk on which appropriate 

 figures or symbols are marked, the velocity of one can be determined if 

 that of the other is known. This principle may possibly be applied with 

 advantage to determine the relative velocity of two machines, such as the 

 twin screws of ships. 



Without pretending that the method described in this paper for deter- 

 mining the velocity of rotation will be useful on an extensive scale, we 

 hope that it may be applicable in some cases of investigation where 

 accurate observations can be rapidly made without any complex apparatus 

 or difficult manipulation. It has the advantage that it can be applied 

 directly to a machine without the intervention of any gearing, the mere 

 attaching of a piece of paper to a shaft being all that is necessary. It 

 cannot, by giving the machine more work to do, produce any effect on 

 its rate; and by the impossibility of slip, it must give accurate results if 

 the paper is properly mounted in the first instance and the observations 

 are properly made. 



Being an optical method for investigating rotation, we suggest cyclo- 

 scope as a name for the instrument. 



April 26, 1877. 



Dr. J. D ALTON HOOKER, C.B., President, in the Chair. 



The Presents received were laid on the table, and thanks ordered for 

 them. 



The following Papers were read : — 



1. " On the Nature and Origin, of the Beds of Chert in the 

 Upper Carboniferous Limestones of Ireland/" By Prof. 

 Edward Hull, M.A., E.R.S., Director of the Geological 

 Survey of Ireland. With " Chemical Notes/' by E. T. Hard- 

 man, F.C.S., of the Geological Survey of Ireland. Received 

 March 16, 1877. 



(Abstract.) 



After reviewing what had been published by previous authors on the 

 origin of chert-beds, and showing that much remained to be done in this 

 department of petrology, the author proceeded to describe the geological 



