196 



Mr. A. E. Donkin on an Instrument for the [Feb. 19, 



TABLE II. 



TABLE III. 



The general result of these experiments is, that sulphate-of-lime solu- 

 tion absorbs a little more carbonic acid than water, but follows the same 

 law of variation with temperature and pressure ; sulphate-of -magnesia 

 solution differs slightly from water when but little time is left for the 

 reaction to complete itself. If, however, the gas and solution are left in 

 contact for a considerable time, the difference between the coefficients 

 of water and of the salt solution becomes very marked, that of the latter 

 being less for high pressures and greater for low ones than that of water. 



The details of these experiments will be found elsewhere in a more 

 extended paper. 



II. " On an Instrument for the Composition of two Harmonic 

 Curves/' By A. E. DONKIN, M.A., F.R.A.S., Fellow of Exeter 

 College, Oxford. Communicated by W. SPOTTISWOODE, Treas. 

 K.S. Received November 6, 1873. 



The interest in such compound curves lies in the fact that as a simple 

 harmonic curve may be considered to be the curve of pressure on the 

 tympanic membrane when the ear is in the neighbourhood of a vibrating 

 body producing a simple tone, so a curve compounded of two such simple 

 harmonic curves will be the curve of pressure for the consonance of the 

 two tones which they severally represent, and thus the effect on the ear 

 of different consonances can be distinctly represented to the eye. 



If the motion of a point be compounded of rectilinear harmonic 

 vibrations and of uniform motion in a straight line at right angles 

 to the direction of those vibrations, the point will describe a simple 

 harmonic curve. 



Thus a pencil-point performing such vibrations upon a sheet of paper 

 moving uniformly at right angles to their direction would draw such a 

 curve. 



The same kind of curve would also be drawn by keeping the pencil 

 fixed and by giving to the paper, in addition to its continuous transverse 

 motion, a vibratory motion similar and parallel to that which the pencil 

 had ; and if the motion of the latter be now restored, a complicated curve 

 will be produced whose form will depend on the ratio of the numbers of 



