342 



N-A TURE 



[August 12, 1897 



LETTERS TO THE EDITOR. 



\The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.'\ 



Cyclone Sail. 



I HAVE sent to you, for publication if you think desirable, a 

 photograph of a type of an ideal sail — ideal, in that the wind 

 acting on it has no tendency whatever to incline the boat. 



The wind pressure acts practically at right angles to the mean 

 surface of the sail. When the wind is making a large angle 



with the sail, the centre of pressure is almost at the centre of 

 the surface, but when the wind strikes the sail at an acute 

 angle, as in all sails or kites, the centre of pressure moves 

 towards the weather edge ; but by suitably adjusting the sail, 

 the desirable result of obliterating all heeling movement has been 

 achieved. 



The training in a horizontal direction is accomplished by 

 I means of a turntable, and the elevating and lowering by two 

 I tackles. 



j There is a balance weight which helps in elevating the mast, 

 ; and which is just sufficient to balance the dead weight of the 

 I sail in a calm, not inclining the boat. 



The sail can be set and furled in a minute ; it does not close 

 like an umbrella, but each side shuts up like a fan. 



The object of the sail is to be able to sail without inclining 

 the boat, so that the limit of driving force is not governed by 

 the stability of the boat in any way, and also that the boat sail- 

 ing on an even keel has less resistance than when sailing with. 

 a list. Percy S. Piixher. 



Artillery Mansions, 75 Victoria Street, S.W. 



A Hertz-Wave Model. 



I In the spring of the present year I showed, at a meeting of 

 ; the Physical Society of London, a wave-motion model which I. 

 ! designed to illustrate mechanically the propagation of a trans- 

 verse wave. As the exhibition of this model on that occasion, 

 and subsequently at the Royal Society and Royal Institution, 

 has elicited a number of inquiries about the apparatus, it is 

 ; thought that the following brief account of it may be of some 

 1 interest to lecturers on physics, particularly at a time when the 

 [ propagation of electric waves through space is occupying much 

 attention. The apparatus, which is depicted in the accompany- 

 ing cut, is mounted on a strong wooden frame about 2 metres 

 long. At one end (the further in the cut) is the " oscillator," a 

 heavy mass of brass hung by two strong V cords from arms 

 which project parallel to the longer dimension of the frame. 

 This mass, which, for the sake of analogy, is quite unnecessarily 

 shaped to imitate an orthodox electric oscillator, can therefore- 

 be set swinging in a transverse direction by a suitable impulse 

 given by hand. At the other end of the frame (the nearer in 

 the cut) is the "resonator," a circle of brass wire hung by a 

 tri-filar suspension. Oscillator and resonator must be adjusted 

 by shortening or lengthening the cords so as to have identical 

 periods of oscillation. The real problem in the construction of 

 the apparatus was to find a mechanical means of transmitting the 

 energy of the oscillator in visible waves to the resonator.. The 



al model illustrating propagation of a Hertz wave. 



In practice this result has been obtained by putting more sail 

 to leeward than to windward of the mast, and also by placing 

 the sail not quite at right angles to the mast, but more raised on 

 the lee side. 



The sail is made oval, with the major axis horizontal, so as 

 to be able to carry more sail with a definite height of mast. 



NO. 1450, VOL. 56] 



means finally adopted was a series of inter-connected pendulums 

 on a plan somewhat similar to one suggested ^ in 1877 by Prof. 

 Osborne Reynolds. Instead of using springs, however, the 

 requisite inter-connection is obtained by simply suspending the 

 leaden bullets which act as pendulum-bobs by V suspensions. 

 1 See Nature,. vol. xvi. p. 343. 



