Elements of natural philosophy (Volume 2-3) . tly clear the reports of their guns were always single and sharp; whereas when the sky wras overcast or a single cloud of any extent was present, they were fre- Experiments; quently accompanied with a long continued roll like that of thunder, and occasionally a double sound would arrive from a single shot. But it is proper to remark that the rolling of thunderadmits of another explanation. Thunder is caused by adisturbance of electrical equilibrium in the atmosphere;experience shows that this takes place over a long andsinuous line, the different p
Elements of natural philosophy (Volume 2-3) . tly clear the reports of their guns were always single and sharp; whereas when the sky wras overcast or a single cloud of any extent was present, they were fre- Experiments; quently accompanied with a long continued roll like that of thunder, and occasionally a double sound would arrive from a single shot. But it is proper to remark that the rolling of thunderadmits of another explanation. Thunder is caused by adisturbance of electrical equilibrium in the atmosphere;experience shows that this takes place over a long andsinuous line, the different points of which are at unequal Roiling ofdistances from the auditor, and the sounds from these thunder-points can, therefore, only reach him in succession andwithout sensible intervals. § 82. When reflected sound and that proceeding di-rectly from the same source, are made to fall upon the 90 NATURAL PHILOSOPHY. Reflected soundmay increase theeffect of directBound; Illustrated bythe speakingtrumpet; Its construciionand useexpiaincd; Fig. Ji^JK B}r its use soundsare renderedaudible thatcould not beheard without it. ear simultaneously, or nearly so, tliey strengthen eachother and become audible in positions where neithercould be heard separately. The Speaking Trumpetaffords an illustration of this. The Speaking Trumpetis a funnel-shaped tube, of which the object is to throwthe voice beyond its ordinary range. In its best formit is parabolic. It is a geometrical propertyof the parabola that a lineFT, drawn from the focusF, to any point T of thecurve, and another T K,drawn from .T7 parallel to theaxis FA, make equal angleswith the tangent line to the ^\ curve at T. A portion of the diverging rays of sounds proceeding from a mouthat the focus F, will be reflected by the trumpet indirections parallel to the axis A F; and the living forcesof the aerial molecules which, without the trumpet,would have been diffused over that portion of the spheri-cal surface on the outside of a con
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