319 



which the length of a needle has on its deviations, produced by the 

 attraction of the shell of iron. When examined by the test of the 

 formulae given by the author, the law of the tangent of the devi- 

 ation being proportional to the rectangle of the cosine of the longi- 

 tude into the sine of the double latitude on which so much depend- 

 ance had been placed, is found to give results so inconsistent with 

 one another, that it cannot be considered as even affording an ap- 

 proximation to the truth, and must therefore be wholly rejected. 



The close agreement which the author found between the ob- 

 served and the computed deviations of needles, whose magnetism had 

 been disturbed by contact with a magnet, as well as those which had 

 suffered no disturbance, fully confirmed the author in the views 

 which he originally took of the action of iron on magnetized needles. 

 He conceives that his hypothesis, instead of being at variance with 

 observation, is not only consistent with all the experiments that have 

 been made, but by affording the proper corrections to be applied to 

 them, derives the strongest support from these observations. 



He concludes by mentioning a fact which he conceives to be irre- 

 concileable with the hypothesis of induced magnetism ; namely, that 

 a steel bar, rendered as hard as it was possible to make it, produced, 

 when its ends are reversed, precisely the same effect on the needle 

 as a bar of the softest iron under similar circumstances. 



Description of a Sounding Board in Atterdiffe Church, invented by the 

 Rev. John Blackburn, Minister of Attercliffe-cum-Darnall, Shef- 

 field. Read June 5, 1828. [Phil. Trans. 1828, p. 361.] 



The church at Attercliffe had long been remarkable for the diffi- 

 culty and indistinctness with which a voice from the pulpit was heard. 

 These defects have been completely remedied by the erection of a 

 concave sounding-board, having the form resulting from half a revo- 

 lution of one branch of a parabola on its axis. It is made of pine 

 wood ; its axis is inclined forwards to the plane of the floor, at an 

 angle of about 10 or 15; it is elevated so that the speaker's mouth 

 may be in the focus ; and a small curvilinear portion is removed on 

 each side, so that the view of the preacher from the side galleries 

 may not be intercepted. A curtain is suspended from the lower edge 

 for about 18 inches on each side. 



The effect of this sounding-board has been to increase the volume 

 of sound to nearly five times what it was before ; so that the voice 

 is now audible, with perfect distinctness, even in the remotest parts 

 of the church, and more especially in those places which are situated 

 in the prolongation of the axis of the paraboloid. But the side gal- 

 leries are also benefited ; probably from the increase of the secon- 

 dary vibrations in a lateral direction. Several experiments are re- 

 lated illustrative of these effects ; among which, the most striking, 

 was one in which a person placed so as to have one ear in the focus 

 of the paraboloid, and the other towards another person speaking 



