177 



time that the red is a yellow red, they hecome quite as distinct to 

 the colour-blind as to the normal-eyed. 



The colouring of geological maps is very perplexing to the colour- 

 blind, and it is recommended that engraved marks, to distinguish 

 the different strata, should always be added to the colours. 



In conclusion, the author gives hints which he considers useful for 

 the examination of colour-blind persons, and states the importance of 

 collecting further evidence on the subject, of an accurate and definite 

 nature. 



II. " Researches on the Velocities of Currents of Air in Vertical 

 Tubes, due to the presence of Aqueous Vapour in the At- 

 mosphere." By W. D. CHOWNE, M.D. Communicated by 

 JOHN BISHOP, Esq., F.R.S. Received May 22, 1856. 



(Abstract.) 



This was a paper supplementary to one presented June 14th, 1855, 

 an abstract of which was published in the ' Proceedings of the Royal 

 Society' for June 21st, 1855. The author having ascertained that 

 an upward current of air becomes established in a vertical tube 

 placed in as quiescent an atmosphere as can be obtained, and having 

 demonstrated its existence by means of anemometric discs placed in 

 tubes as described in that paper, proceeded to ascertain the velocity 

 of the currents by which the discs were moved. 



In order to estimate the velocity of the currents, one of the ane- 

 mometric discs was placed within a short zinc tube three inches in 

 diameter, the lower end of which was accurately fitted into an aspi- 

 rator capable of containing thirty-six gallons of water. By drawing 

 off in a given time a quantity of water equal in bulk to the cubic 

 contents of one of the tubes described in the former paper, the 

 velocity of a current required to produce a given number of rotations 

 of the disc was determined. 



The experiments were varied by altering the height of water in 

 the aspirator, and thereby changing the velocity, while the exit- 

 orifice remained unaltered. 



By ascertaining the number of rotations of the anemometric disc, 



