i:. 





Explana- 

 tion ot the 

 deception of 

 the invisible 

 girl. 



PLATE 



tX'CCI.XXXIY 

 L'ig. 1. 



case, as the hun inin j "f insects wns much gronter in tljc 

 ' il ' c * night than in tli<- iuv, and tin- !<r y.r, whieh might 

 have agitated th-- Icavo. ol'the tree?, never roe till after 

 l "" v ~" sunset. Ilumboldt, therefore, was led to ascribe the 

 diminution of the sound during the day to the presence 

 of the sun, whirh influences the propagation and inten- 

 itv of .sound, by opposing to them currents of air of 

 different density, and partial undulations of the atmo- 

 sphere, produced by unequal heating of the different 

 parts of the ground. In these cases, a wave of sound, 

 when it meets two portions of air of different density, 

 is divided into two or more waves, a part of the primi- 

 tive wave being propagated with more rapidity through 

 the denser portions, than the parts that pass through 

 air of less density. In this way the wave is broken down 

 into different parts which arrive at the ear at different 

 times. These different portions of the wave passing 

 again through succeeding portions of the atmosphere 

 of different density, may be so wasted and frittered 

 down, as to be incapable of affecting the tympanum. 



This curious phenomenon is precisely analogous to 

 the production of the mirage, or phenomena of unequal 

 refraction, which are occasioned by the mixture of por- 

 tions of air of different refractive density. 



5. Explanation of the deception of the invisible Girl. . 



In our article ACOUSTICS, Vol. I. p. 127, we have 

 mentioned in a few words the principle of this very 

 singular deception, which was some years ago exhibit- 

 ed in London and Edinburgh by M. Charles. 



A perspective view of the apparatus by which 

 this deception was performed, is shown in Plate 

 CCCCLXXXIV. Fig. 1, a plan of it in Fig. 2, and 

 a section of it in Fig. 3. 



The apparatus shown in Fig. 1, was suspended in 

 the middle of a room lined with wainscot, though this 

 is not essential to the experiment. Four upright posts 

 A, A, A, A, Fig. 1, are united at top by a cross rail 

 BB, and by two similar rails at bottom. This frame- 

 work being placed on the floor, there proceeded from 

 the top of each upright post A, four bent cross wires 

 a, a, a, a, which met together at the top c, where they 

 terminated in a summit of any kind. From these four 

 wires a hollow copper ball M, about a foot in diame'er, 

 was suspended by four slender ribbons b, b, b, b ; and 

 round this ball were placed four trumpets T, T, T, T, 

 having their mouths opening externally. 



When the spectator entered the apartment, he was 

 called upon by the exhibitor to propose some question, 

 which he did by speaking into the mouth of one of the 

 trumpets T. When this was done, an answer imme- 

 diately issued from all the trumpets, sufficiently loud to 

 be heard by an ear applied to any of them, and yet so 

 weak that it seemed to come from a very diminutive 

 person. The invisible lady conversed in several differ- 

 ent languages, sung beautifully, and made the moit 

 lively and apposite observations on whatever was go- 

 ing on in the room. 



The method by which this deception was carried on 

 b shown in Fig. 3. One of the posts A A, and one 

 half TB of the hand-rail connected with it is hollow- 

 ed into a tube, one end of which appears on the inside 

 of the railing exactly opposite to the mouth of the trum- 

 pet T, while the other end communicated with a tube 

 p p going below the floor J'J\ and passing up the wall 

 to a wide deal case h k, similar to nn inverted fun- 

 nel, and large enough to contain a lady with a piano 

 forte. A small hole closed with glass, is left through 

 the funnel and side wall of the room about h, through 



VOL. XVII, PART II. 



Fij. 3. 



which the Isdy may oboerve what it going on among Sclent* 

 tin- .-Mid: tort. * 



U lu-n a question i< now avkcd nt the mouth of one '* 

 of the trumpets T, the sound patte* through th* tube * """*"* 

 TAA j>i> h into the fur.nel /<K, at.d i* therefore ditti! 

 ly heard by the lady. The answer likewise parses from 

 die funnel along the tube h p n A AT, and btriking the 

 mouth of the trumpet, it it reflected back from it to the 

 car of the auditor. The sound appears to issue also 

 from every trumpet as the tubes communicate* with 

 each other. 







(]. On Venlrilnqnisnt. 



In our article ACOUSTICS, Vol. I. p. 128, we have On Veturi- 

 given a short notice on the subject of ventriloquism ; l 

 but as the explanation there given is imperfect, though 

 correct so for as it goes, it is proper to return to the 

 subject. 



The art of ventriloquism is founded upon a diligent 

 study of th.- modifications which sounds undergo, 

 when emitted under a variety of different circumstan- 

 ces ; and, without a perfect knowledge of this part of 

 his art, the ventriloquist will display his powers with 

 little success. A sentence uttered by the same lips, 

 and with the same intonation, behind a door shut or 

 half open, or by a person enclosed in a box, or secreted 

 in the chimney, will obviously convey to those who 

 hear it some idea of the locality of the person who ut- 

 ters it. If a ventriloquist, therefore, should have stu- 

 died these modifications so completely, that he can pro- 

 nounce the sentence with the precise modifications 

 which the sound experiences, those who hear this sen- 

 tence must believe that it is uttered by a person in the 

 chimney for example. In order, however, that ibis be- 

 lief may be complete, the ventriloquist must turn his 

 back upon his auditors, or must possess also the power 

 of speaking with the muscles of his throat, so as not to 

 move his lips, or alter the features of his face ; for if 

 any muscular action were seen in the face of the per- 

 former, it will be a vain attempt to impose upon the 

 auditors, however nicely the sound be imitated. The 

 only conclusion would be, that the performer was an 

 excellent imitator, but without any powers of deception. 



But even if the ventriloquist has the power of speak- 

 ing without moving the muscles of his throat or face, 

 and has the most complete power of imitating sounds 

 under all possible modifications, another condition is 

 necessary to the success of his performances. A line 

 drawn from the mouth of the ventriloquist to the ear 

 of any of his auditors, must not be greatly inclined to 

 the line drawn from the object from which he wishes 

 the sound to appear to proceed to the ears of any ot" 

 hi:; auditcrs. If t!~e ventriloquist, for example, were 

 placed to the south of his auditors, it would be in vain 

 tor him to attempt to cause any sound to proceed from 

 an object to the north of the auditors, or even from an 

 object east or west of them. The dullest ear is capable 

 of distinguishing the direction from which sounds actu- 

 ally proceed to a greater degree than this. There is, 

 however, a certain angle within which the ear cannot 

 distinguish differences in the direction of sounds. Thus 

 if a sound issued exactly from the south point of the 

 horizon, and the same sound from a point of the com- 

 pass to the west of south, an ordinary ear could not 

 determine which of the two sounds came from the 

 south, and which from the west of south. The ventri- 

 loquist must therefore take care not to place the object, 

 from which the sounds are to appear to come, without 

 the range of this angle. 



4B 



