all known Substances to the Magnetic Injltience, 4'c- ^29 



of the opposite poles of the magnets, and especially if each maxi- 

 mum be itself determined by the mean of equal deviations on 

 both sides of the maximum ; in this case I have frequently de- 

 termined, at moderate distances, the very point on the cross 

 plank, from whence a line drawn at right angles will pass through 

 the magnets, consequently the nearest possible distance between 

 the magnets and compass. Where the deviations, however, are 

 very small, as it will then require a considerable change in the 

 position of the compass, laterally, to produce any sensible change 

 in the deviation, — the line of direction, or the level, will be pro- 

 portionally less certain, though the nearest distance may be suf- 

 ficiently ascertained. 



As the person who conducts any experiments on the foregoing 

 principles will have to direct the application of the magnets, 

 with any changes that may be requisite as to their poles or posi- 

 tion, whilst the experiment is in progress, an arrangement of 

 directions by signal will be necessary ; and this may be very 

 well accomplished by the blows of a hammer or mall. 



The distance at which any sound can be heard through solid 

 substances, will depend both on the nature of the concussion, 

 and the quality of the vibrating mass. Through earth, gravel, 

 or other loose materials, the transmission of sound is very li- 

 mited and uncertain ; but through solid uniform rock, direc- 

 tions may be communicated by signals to very considerable dis- 

 tances. In solid and uniform sandstone, the pick, I understand, 

 may be very well heard through a distance of fifteen or sixteen 

 yards, whilst the vibration produced by a blast may be percei- 

 ved at very great distances beyond. The working of coal by 

 the pick may be generally heard sixty or eighty feet, I am told ; 

 though no rational determination of the thickness can be made 

 from the nature or intensity of the sound, — for a change in the 

 structure of the bed, or the interposition of an increased quan- 

 tity of moisture, may produce as great an alteration in the sound 

 as might be expected if the distance were doubled ! The blow 

 of agnail upon an iron wedge set in the rock, may be heard 

 much farther than the stroke of a pick, or the blow of the hea- 

 viest hammer on the face of the rock, — and this will form a 

 convenient arrangement for the purpose of signals. 



VOL. xiir. Ko. XXV. — JULY 1832. i 



