322 



NATURE 



[August 6, 1903 



an abundance of good hardwood already in the country, and 

 the four pitch-pines, rivalling hardwoods in strength and 

 durability. Eucalypt culture in America is still in its 

 infancy ; they have not yet discriminated the valuable from 

 the many worthless species, nor fitted, as far as may be, the 

 species to its climate. D. E. Hutchins. 



E. Hutchins. 

 Cape Town, June 23. * 



A Simple Form of Tide Predictor. 



For the past four years a very simple form of tide-pre- 

 dicting machine, the invention of Captain A. Inglis, the 

 harbour-master, has been in use at Port Adelaide for the 

 construction of the yearly published tide tables. The tides 

 at Port Adelaide are rather peculiar in their behaviour, this 

 being' due principally to the fact that the solar and lunar 

 semi-diurnal components are almost exactly equal. At and 

 near the neaps these neutralise one another, and the diurnal 

 components, which are relatively large, are then the main 

 sources of the tidal movement. Before these tides were 

 harmonically analysed, their prediction by ordinary methods 

 was quite impossible, except near the springs. By means 

 of this machine, however, they are now predicted yearly 

 with considerable accuracy. The essential principles of the 

 machine are as follows : — A number of thin wooden templets 

 are cut, each in the form of a sine curve, representing the 

 various tidal components (Fig. i). These waves are of 

 different lengths, the length of each component wave bearing 



carries a sheet of paper on which the tidal curve may be 

 traced if required (P^ig. 2). 



Each templet is fixed in the carrier in proper relative 

 position according to its phase at the start, as determined 

 by previous harmonic analysis. When the handle of the 

 machine is turned, the carrier, vertical slide and clock are 

 set in motion, and the indicator shows the height of the 



j tide at the time shown by the clock, and the curve may at 

 the same time be traced on the vertical slide. 



There are three carriers and three or four templets to 

 each component. When one of the carriers has been worked 

 forward far enough, it can be disconnected from the others 

 and connected up again at the other end. The curves are 

 again placed in their respective grooves, and, by means of 

 a suita-ble attachment, butted close up to the preceding ones. 

 In this way the process is made continuous. 



The setting of the curves can easily be checked at every 

 month, to see that there has been no slipping. 



The machine involves no expensive construction, and 

 enables a year's tides to be predicted expeditiously, and, as 

 experience has proved, with quite sufficient accuracy. 



The University, Adelaide. R. W. Chapman. 



[In a subsequent letter, Mr. Chapman informs us that 

 he made the following errors in the list of values of the 

 tidal components at Port Darwin, printed in last week's 

 j Nature (p. 295). " The amplitude of N should be 104, of 

 V 048, and of T 1-53. The phase of v should be 141°, and 

 that of T 70°." — Ed. Nature.] 



the same ratio to the solar semi-diurnal as its angular speed 

 does to 15°. The templets are all fixed side by side, with 

 their planes vertical and parallel, being supported on a 

 carrier, which can be moved forward in the direction of the 

 waves by means of a rack and pinion underneath. A 

 number of vertical plungers rest in a transverse line with 

 their lower ends resting on the tops of these templets, and 

 are moved up and down as the curves progress forward. 

 The motions of the plungers are then compounded by means 

 of a fine wire passing over pulleys at the top of each one, 

 and under fixed pulleys between adjacent ones. This wire 

 is connected to an indicator, which moves up and down: 

 alongside a vertical scale, thus marking the height of the 

 compound wave at any instant. 



The wire passing over the plungers is an endless wire, 

 going round a pulley on the indicator and round a larger 

 pulley at the other end of the line of plungers. This larger 

 pulley is attached to a plate which is movable backwards 

 and forwards by means of a fine screw. This gives a means 

 of adjusting the height of the indicator, and also of allow- 

 ing for the effect of the annual and semi-annual tides. 

 The rise or fall due to these long period tides is treated 

 as constant for fourteen days, and the screw adjusted so 

 as to alter the height of the indicator by the proper amount 

 at the end of each such interval. In front of the frame of 

 the machine, between it and the indicator, is a vertical slide, 

 which is moved forward at the same rate as the carrier, and 



NO. 1762, VOL. 68] 



Sympathetic Song in Birds. 

 • In your issue of April 30 (vol. Ixvii. p. 609) Mr. George 

 Henschel describes an interesting vocal duet between a bull- 

 finch and a canary, and invites contributions to the subject. 



In 1893 I obtained a nestling Australian magpie 

 (Gyninorhma tibicen, Latham), and taught it on the flute to 

 pipe the following : — 



.Some years later I acquired another bird of the same 

 species ; this learned the tune from the original magpie. I 

 do not know how the birds agreed upon the duet (or fugue) 

 rendering, but it was performed in the following way : — 

 When the first bird commenced its song, the second one 

 immediately came to attention, and with half-open beak 

 avv'aited the point marked *, whence it finished the strain 

 alone. The birds were kept in a large outdoor aviary in 

 company with many others, and no matter where or how 

 engaged, the second bird would, on hearing its mate, 

 assume an attentive attitude, and await the conclusion of 

 the first portion of the theme. 



The second bird died, and the original one, which I still 

 have, now pipes the whole strain alone, as was its original 

 custom. 



I may also mention that this bird has the faculty of 

 absolute pitch, and pipes the theme in F as originally 

 taught. Edgar R. Waite. 



Australian Museum, Sydney, June 18. 



