NATURAL PHILOSOPHY. 255 



from storms or squalls, or in mines from the accumulation of carburctted 

 hydrogen gas, the pointer will come in contact with the metallic danger-point 

 in the face of the dial, and so complete the electric circuit which would have 

 the effect, by the intervention of the ordinary or well-known apparatus of 

 sounding an alarum, either by ringing a bell or exploding gunpowder. The 

 arrangements may be varied to suit the particular case, by making the dial 

 of some conducting material and the danger-points of non-conducting mate- 

 rial, so that it would be the breaking of the electric circuit that would give 

 the alarm. In the case of ships the barometer might be placed in the captain's 

 cabin with an alarum there, and another on deck near the wheel. While in 

 mines a barometer should be placed in every gallery with an alarum to warn 

 the miners who might be there ; and it should also be in communication with 

 an alarum and corresponding dial indicator at the mouth of the pit, so that the 

 overlooker might at all times know the state of the atmosphere in every part 

 of the mine, and be warned of the first approach of danger. 



Guynne's Barometer. This new invention consists of an inverted glass 

 syphon, sealed at one end and open at the other, partly filled with mercury, 

 and suspended, so that the two limbs of the syphon form the arms of a bal- 

 ance. The invention consists chiefly in supporting or balancing the instru- 

 ment on points, pivots, or knife-edges, or suspending it by a flexible material, 

 as a silken cord, a fine flexible steel spring, c., which allows the instrument 

 to vibrate or oscillate freely, and a pointer or hand fixed to the instrument, 

 and moving in front of an index or dial, shows by its motion the most 

 minute change in the atmosphere. Any increase in the pressure of the at- 

 mosphere forces the mercury towards the scaled end of the tube, giving a 

 preponderance to that side of the syphon, and, consequently, motion to the 

 instrument ; while a decrease in the pressure produces an effect in the 

 opposite direction. A great many varieties in the arrangement of the in- 

 strument may be made. 



CHRONOMETER COMPASS. 



By means of this instrument, which is a combination of a universal dial and 

 chronometer, the inventor, Mr. Ralph Reeder, of Cincinnati, claims to be 

 enabled to take any horizontal bearing, in any latitude, at any time of the 

 day, by bringing the shadow of the gnomon to its proper place. The gno- 

 mon revolves by means of the chronometer, so as to perform one revolution 

 in twenty-four hours ; and when the instrument is levelled and elevated to 

 true latitude, and adjusted at the meridian, the gnomon points steadily to 

 the sun, which it follows in its course. And conversely, if the instrument be 

 levelled and elevated to the latitude of the place, and turned round horizon- 

 tally till the gnomon points to the sun, or till the shadow falls on the proper 

 point, it will be adjusted to the meridian, and an angle or bearing may be 

 laid off by a horizontal gradual motion. It will also solve practically all 

 the problems which can be solved by any armillary sphere, or by spherical 

 trigonometry, so far as its circles and its motions extend. Thus, the decli- 

 nation and the time given, it will show the altitude and the latitude at any 

 hour and at any place. The instrument is constructed on correct mathe- 



