194 



KNOWLEDGE. 



[September, 1901. 



eleven p.m. These service signals are uniform in 



character, and the first, though of the usual intensity, 

 was followed by little after-sound. After an interval 

 occupied only by the burning of two distress signals 

 another gun-cotton charge was fired from within a yard 

 or two of the same spot, the reverberations of this. 

 however, were so peculiar and prolonged as to disturb 

 the neighbourhood. If the great difference of after- 

 sound was not due to the slight alteration of position, 

 it can only be refened to the fact that the first cartridge 

 was fired eighteen inches above the ground, while the 

 second was laid on the hard earth, in which it blew a 

 deep cavity. 



With reference to the far hearing of bells one most 

 important statement to make is that their sound is 

 extremely uncertain. Seamen, lighthouse keepers and 

 others, whose training makes them close observers, 

 constantly insist on this. I have already shown how 

 remarkably the sound of a bell may be lost in the free 

 upper ah-. Against this may be quoted a statement 

 which I have on the authority of Messrs. Mears, the 

 well-known bell-foundei-s. It appears that the tenor bell 

 of the peal of St. Bees, on the coast of Cumberland, has 

 been heard at the top of Scafell Pikes, sixteen miles 

 distant in a straight line. This is certainly a record, 

 and must probably be largely accounted for by the slope 

 of the mountain. My own experience is wholly against 

 the possibility of the hearing of a deep bell across such 

 distance and at the height of 3000 feet in the free 

 heaven. The stoiy of the sentry on duty on the terrace 

 of Windsor Castle suspected of drowsiness and yet 

 detecting the clock bell of St. Paul's striking thii-teen 

 is quite incredible to one who has frequently and from 

 chosen places of advantage listened to the clocks of 

 London striking midnight. To begin with, hundreds of 

 other clocks are striking at the same period, and for 

 anyone to accurately count out the strokes of any 

 particular bell would require that observer to be very 

 wide awake indeed, and certainly at a nearer distance 

 than twenty odd miles. But a very interesting' and 

 instructive series of observations relative to the heai'ing 

 of a bell heavier by ten tons — namely. Big Ben — has 

 been contributed to the Quarterly Journal of the Roy. 

 Met. Soc. by Mr. William Marriott. This observer, 

 making careful note twice daily from a station in West 

 Norwood, only five and a half miles away, records that 

 the bell was very distinctly heard four times, faintly 

 heard 64 times, and altogether unheard on 251 

 occasions. A significant comment is added to the effect 

 that the most favourable hours were those of evening 

 and Sundays. 



Where a bell is chosen for a warning signal, e.g., as 

 on a lighthouse, and it is desired that its sound be 

 carried in all directions over as great horizontal ranges 

 as possible, the most efficient form of sound-board will 

 be found to be that shown in the accompanying figure. 

 Here it will be seen by taking any vertical section of 

 the apparatus that every point on the sound-bow of 

 the bell (i.e., that zone that is in most intense vibration) 

 virtually occupies the focus of a parabolic reflector, and 

 thus the principal sound waves ai-e made to travel out^ 

 wards horizontally in parallel rays. 



An approximation to the jiaraboloid will be found 

 the best possible fomi for installments designed alike to 

 convey and receive the human voice at long range. 

 Obviously some little compromise in outline will be 

 necessary. The aperture of the car in listening, as also 

 the lips in speaking, should, as nearly as possible, occupy 



the focus of each instrument. Thus it is clear that 

 the lat-tis rectum should not much exceed li inches, and 

 it will then be found convenient that the larger end of 

 the instrument (which may mea.sure some 14 inches in 

 length) should be somewhat constricted so as to 

 approach a cylindrical form. With a pair of these 

 instruments, forming respectively a giant speaking 

 trumpet and a giant ear, speech can be carried on across 

 an open common on a quiet night over a mile range. 

 Any appreciable amount of wind stirring affects results 

 gi-eatly. A favouring wind scarcely favours the hearing 

 of the voice, doubtless largely owing to the muraiur in 

 the air caused by the wind stirring the herbage, etc. 



It may be otherwise in the case of a gun. Dr. Davison 

 considers that piesuming the velocity of wind to 

 increase with height the report of a gun might be 

 audible at a much greater distance in wind than in calm. 

 Tyndall points out that in wind a gunshot may readily 

 be lost altogether, but states as an observation of his 

 own that on a windy day a gun was heard five times 

 and might probably have been heard fifteen times, as 

 far to leewai'd as to windward. 



It has been suggested that the peculiar loudness of 

 some of the repoi-ts heard on February 1st may have 

 been due to the discharge of several guns at practically 

 the same instant. This argument may be valid enough 

 in the case of independent but simultaneous gun firing, 

 but it is only true in a very modified degree in the case 

 of the usual gun-cotton fog-signals already spoken of, 

 and chosen by the service for special penetration. The 

 explosion of an eight>-ounce cartridge is certainly not 

 nearly twice as loud as that of four ounces, while 

 through interference or some other cause the firing of 

 two four-ounce cartridges in juxtaposition is not very 

 greatly louder than that of one. On an occasion when 

 experimenting on echoes I was firing a number of these 

 fog-signals singly from the clouds I designed to' make a 

 superlative discharge by firing a nest of many united 

 together. It chanced that I was able to explode this 

 giant bomb nearly over the racecourse at Epsom at an 

 altitude of about half a mile, the course was deserted, 

 but the nature of the ground seemed favourable for a 

 gi-and effect. I can but state, however, that the result- 

 ing report and subsequent echoes were under the 

 circumstances disappointing. 



THE INSECTS OF THE SEA.-V. 



By Geo. H. Caepenter, b.sc.(lond.), A.fsistant in the 

 Museum of Science and Art, Dublin. 



FLIES. 



Two-wiNGED flies are perhaps the most dominant of all 

 insects. The woodland rambler in summer-time knows 

 too well what swarms of flies hover with ceaseless 



