102 HARMONIC SLIDERS. 



preffions in confequence of the fecond, and fubtract them when 

 they counteract the effect of the firft : or we may add the whole 

 height of the fecond above any given point or line, and then 

 fubtracr, from all the funis, the diftance of the point affumed 

 below the medium. 



Exhibition of a To do this mechanically is the object of the harmonic Aiders. 



fimple tide, The f ur f ace f t he firft tide is reprefented by the curvilinear 

 termination of a tingle board, Plate VI. Fig. 1. The fecond 

 tide is alfo reprefented by the termination of another furface ; 

 but, in order that the height at each point may be added to the 

 height of the flrft tide, the furface is cut tranfverfely into a 

 number of feparate pieces or Aiders, which are confined within 

 a groove or frame, and tightened by a fcrew, Fig. 2. Their 



and of combina- lower ends are fituated orginally in a right line ; but, by 



tions which e.- ] 00 f en ; n or the fcrew and moving the Aiders, they may be made 



ther increafe, or ° & . 



modify, or de- to affume any other form : thus they may be applied to the 

 ftroy each other. f ur f ace reprefenting the firft tide ; and if the fimilar parts of 

 each correspond, Fig. 3, the combination will reprefent a tide 

 of twice the magnitude of the fimple tides. 



The more the correfponding parts are feparated, the weaker 

 will be the joint effect, Fig. 4; and, when they are furtheft 

 removed, the whole tides, if equal, will be annihilated, Fig. 5. 

 Thus, when the general tide of the ocean arrives by two differ- 

 ent channels at the fame port, at fuch intervals of time that 

 the high water of one would happen at the fame inftant with 

 the low water of the other, the whole effect is deftroyed, ex- 

 The principle is cept fo far as the partial tides differ in magnitude. The prin- 



apphcable to a • | e De j n g once underftood, it may eafily be applied to a mul- 

 vanety of cafes, .' ° . / , y , , . .,.-,•. 



tiphcity or caies : for lnftance, where the undulations differ in 



their dimenfions with regard to extent. Thus, the feries of 

 Aiders being extended to three or four alternations, the effect 

 undulations dif- of combining undulations in the ratio of 2 to 1 of 3 to 1, of 2 to 

 fering in extent. 3^ f 3 to ^ ma y b e afcertained, by making a fixed furface, 

 terminating in a feries of curves, that bear to thofe of the Aiding 

 furface the ratio required : and, by making them differ but 

 Thebeatofan (lightly, the phenomenon of the beating of an imperfect unifon 

 imperfect uni- j n m ufic may be imitated, where the joint undulation becomes 

 alternately redoubled and evanefcent. In Fig. 6, the pro- 

 portion is that of 17 to 18,' and the curvilinear outline repre- 

 sents the progrefs of the joint found from the greateff degree of 

 intenfity to the leaft, and a little beyond it. 



VII. Obfervations 



