HARMONIC SLIDERS. 101 



VI. > 



Aii Account of Dr. Young's Harmonic Sliders. From his 

 Paper in the Journals oftltt Royal Injlitution, p, 261. 



J- HE combination of undulations, however cautioufly the Utility of the » 



world may adopt its application to the explanation of optical Jj^J' undulaT 1 * 



phenomena, is of acknowledged utility in illuftrating the phe-tions in explain- 



nomena of mufical confonances and diffonances, and of unde- ' n & cotemporary 

 •II- . . r , i .founds} and the 



niable importance in accounting for many of the phenomena or tides. 



the tides. Each tide is an undulation on a large fcale ; and, 

 fuppofing the general form of the ocean, in confequence of the 

 attraction of a diftant body, to coincide with that of an oblong 

 fpheroid, as it is found by calculation to do, the feclion of the 

 furface of each tide, if conceived to be unbent from the circu- 

 lar form and extended on a plane, would form the harmonic 

 curve. (Young's Syllabus, IV. 151. 155.) It is remarkable 

 that the motions of the particles of the air in found have been 

 generally fuppofed in theory to correfpond with the ordinates 

 of this fame curve, and that there is alfo experimental reafon to 

 believe, that the pureft and moil: homogeneous founds do in fact 

 agree very nearly with the law of this curve. It is therefore by 

 far the moll: natural as well as the moll: convenient to be af- 

 fumed, as reprefenting the flate of an undulation in general ; 

 and the name of thefe harmonic Aiders is very properly de- The harmonic 

 duced from the harmonic curve. Aiders 



By means of this inftrument, the procefs of nature, in the vifibly exhibit 

 combinations of motion which take place in various cafes of n-,^™^ ^ 

 the junction of undulations, is rendered vifible and intelligible, junction of un- 

 vvith great eafe, in the moft complicated cafes. It is unne- dulatl0ns# 

 cetfary to explain here, how accurately both the fituations 

 and motions of the particles of air, in found, may be repre- 

 fented by the ordinates of the curve at different points : it is 

 fufficient to confider them as merely indicating the height of 

 the water conflituting a tide, or a wave of any kind, which 

 exifts at once in its whole extent, and of which each point 

 paffes alfo in fucceffion through any given place of obferva- • 

 tion. We have then to examine what will be the effect of two Explanation ap- 

 tides, produced by different caufes, when united. In order phed t0 tuics * 

 to reprefent this effect, we muft add to the elevations or de, 

 preffions in confequence of the firft tide, the elevations or do- 



preffions 



