May 8, 1902] 
minimura. The movement of the screen required to pass over 
ten intervals from minimum to minimum may be measured, and 
gives at once the length of five complete progressive waves. 
For the bird-call used in the experiments of this lecture the 
wave-length is 2 cm. very nearly. 
When the sound the wave-length of which is required is not 
maintained, the application of the method is, of course, more 
difficult. Nevertheless, results of considerable accuracy may 
be arrived at. A steel bar, about 22 cm. long, was so mounted 
as to be struck longitudinally every two or three seconds by a 
small hammer. Although in every position the flame shows 
some uneasiness at the stroke of the hammer, the distinction of 
loops and nodes is perfectly evident, and the measurement of | 
wave-length can be effected with an accuracy of about I per 
cent. In the actual experiment the wave-length was nearly 
3.cm. 
The formation of stationary waves with nodes and loops by per- 
pendicular reflection illustrates interference to a certain extent, 
but for the full development of the phenomenon the interfering 
sounds should be travelling in the same, or nearly the same, 
direction. The next example illustrates the theory of 
Huyghens’ zones. Between the bird-call and the flame is placed 
a glass screen perforated with a circular hole. The size of the 
hole, the distances and the wave-length are so related to one 
another that the aperture just includes the first and second zones. 
The operation of the sounds passing these zones is antagonistic, | 
and the flame shows no response until a part of the aperture is 
NATURE 
43 
even 20 from the axial line entails a considerable loss, to be 
further increased as the deviation rises to 4o° or 60°. The 
| difficulty thence arising is met, in the practice of the Trinity 
IIouse, by the use of two distinct sirens and horns, the axes of 
the latter being inclined to one another at 120°. _Inthis way an 
arc of 180° or morecan be efficiently guarded, but a more 
equable distribution of the sound from a single horn remains a 
desideratum. 
Guided by the considerations already explained, I ventured te 
recommend to the Trinity House the construction of horns of 
novel design, in which an attempt should be made to spread the 
sound out horizontally over the sea, and to prevent so much of 
it from being lost in an upward direction. The solution of the 
problem is found in a departure from the usual circular section 
and the substitution of an elliptical or elongated section, of 
| which the short diameter, placed horizontally, does not exceed 
the half wave-length ; while the long diameter, placed vertically, 
may amount to two wave-lengths or more. Qbliquity in the 
horizontal plane does not now entail much difference of phase, 
but when the horizontal plane is departed from, such differences 
enter rapidly. 
Horns upon this. principle were constructed under the super- 
vision of Mr. Matthews, and were tried in the course of the recent 
experiments off St. Catherine’s. The results were considered 
promising, but want of time and the numerous obstacles which 
beset large-scale operations prevented an exhaustive examination. 
On a laboratory scale there is no difficulty in illustrating the 
action of the elliptical horns. They may be made of thin sheet 
brass. In one case the total length is 20 cm., while the dimensions 
| of the mouth are 5 cm. for the long diameter and 14 cm. for the 
Fic. 1. 
blocked off. The part blocked off may be either the central 
circle or the annular region defined as the second zone. Ineither 
case the flame flares, affording complete proof of interference of 
the parts of the sound transmitted by the aperture. 
From a practical point of view, the passage of sound through 
apertures in walls is not of importance, but similar considerations 
apply to its issue from the mouths of horns, at least when the 
diameter of the mouth exceeds the half wave-length. The various 
parts of the sound are approximately in the same phase when they 
leave the aperture, but the effect upon an observer depends upon 
the phases of the sounds, not as they leave, but as they arrive. If 
one part has further to go than another, a phase discrepancy sets 
in. Toa point in the axis of the horn, supposed to be directed 
horizontally, the distances to be travelled are the same, so that 
here the full effect is produced, but in oblique directions it is | 
otherwise. When the obliquity is such that the nearest and 
furthest parts of the mouth differ in distance by rather more than 
one complete wave-length, the sound may disappear altogether 
through antagonism of equal and opposite effects. In practice 
the attainment of a complete silence would be interfered with by 
reflections, and in many cases by a composite character of 
sound, viz. by the simultaneous occurrence of more than one 
wave-length. 
In the fog signals established on our coasts, the sound of | 
powerful sirens issues from conical horns of circular cross-section. 
The influence of obliquity is usually very marked. When the | 
sound is observed from a sufficient distance at sea, a deviation of | 
NO. 1697, VOL. 66] 
| height at which the board is held. 
shorter diameter. The horn is fitted at its narrow end to A 
(Fig. 1), and can rotate about the common horizontal axis. 
When this axis is pointed directly at the flame, flaring ensues; 
and the rotation of the horn has no visible effect. If now, 
while the long diameter of the section remains vertical, the axis 
be slewed round in the horizontal plane until the obliquity 
reaches 50° or 60°, there is no important falling off in the response 
of the flame. But if at obliquities exceeding 20° 
or 30° the horn is rotated through a right angle, 
so as to bring the long diameter horizontal, the 
flame recovers as if the horn had ceased sound- 
ing. The fact that there/is really no falling off 
may be verified with the aid of a reflector, by which the sound 
proceeding at first in the direction of the axis may be sent 
towards the flame. 
When the obliquity is 60° or 70°, it is of great interest to observe 
| how moderate a departure from the vertical adjustment of the 
longer diameter causes a cessation of effect. The influence of 
maladjustment is shown even more strikingly in the case of a 
larger horn. According to theory and observation, a serious 
falling off commences when the tilt is such that the difference of 
distances from the flame of the two extremities of the long diameter 
reaches the half wave-length—in this case 1 cm, It is thus 
abundantly proved that the sound issuing from the properly 
adjusted elliptical cone isconfined toa comparatively nacrow 
belt round the horizontal plane and that in this plane it covers 
efficiently an arc of 150° or 160°. 
Another experiment, very easily executed with the apparatus 
already described, illustrates what are known in optics as 
Lloyd’s: bands. These bands are formed by the interference 
of the direct vibration with its very oblique reflection. If the 
bird-call is pointed toward the flame, flaring ensues. It is only 
necessary to hold a long board horizontally under the direct line 
to obtain a reflection. The effect depends upon the precise 
In some positions the direct 
and reflected vibrations cooperate at the flame, and the flaring is 
more pronounced than when the board is away. In other 
positions the waves are antagonistic, and the flame recovers as if 
no sound were reaching it at all. This experiment was made 
many years ago by Tyndall, whoinstituted it in order to explain the 
very puzzling phenomenon of the ‘‘silent area.” In listening 
to fog signals from the sea il is not unfrequently found that the 
signal is lost at a distance of a mile or two and recovered at a 
greater distance in the same direction. During the recent ex- 
periments, the Committee of the Elder Brethren of the Trinity 
House had several opportunities of making this observation. 
That the surface of the sea must act in the manner supposed by 
Tyndall cannot be doubted, but there are two difficulties in the 
way of accepting the simple explanation as complete. According 
to it the interference should always be the same, which is 
