NATURAL PHILOSOPHY. 159 



the diurnal motion, -was moved somewhat to the eastward. The experi- 

 ment had hitherto been performed with one pendulum ; but, in the 

 present instance, two pendulums were used, and were suspended at a 

 sufficient distance apart to allow of the free vibration of each. The 

 weights were held together by a thread, which, on being burned, 

 released them, so that they were set vibrating, initially, in the same 

 vertical plane ; consequently, to the eye of an observer situated in that 

 plane, the two pendulum wires appeared co-incident, one of them cov- 

 ering, or eclipsing, the other. In a short time, however, the course of 

 the two pendulums visibly altered. As their planes of oscillation 

 appeared to revolve the same way on the earth's surface, the wires no 

 longer covered each other, but appeared to separate and alternately to 

 cross each other. The advantages of this mode of operating were, 

 first, the rapidity with which the deviation of the pendulums was man- 

 ifested ; for, as their planes revolved in the same apparent direction, 

 their arcs diverged from each other twice as fast as either from its 

 initial position ; and, secondly, the apparent crossing and recrossing 

 of the wires constituted, to the naked eye, a much more distinct and 

 palpable test of the result than the apparent motion referred to a plane 

 beneath one pendulum. London Athcn&um. 



BOURDON'S DIPROVEMENTS IN GAUGES, BAROMETERS AND THERMOM- 

 ETERS. 



THE. following is the principle of construction of M. Bourdon's im- 

 proved gauges, barometers, thermometers, &c., which of late has 

 attracted considerable attention : 



It appears that if a brass or thin sheet-iron or steel tube be nearly 

 flattened, and afterwards coiled, the effect of an inward pressure of 

 steam or water is to force it towards its original shape ; the first effect 

 produced being that of tension towards elongation, whether the flat- 

 tened tube be coiled or twisted ; and a contrary effect is produced by 

 unresisted exterior pressure. Thus in shaping it, as we have said, a 

 certain degree of elasticity having been given to the metal, as long as 

 it is not absolutely forced beyond a given point of its acquired shape, 

 it will act as a spring to the greatest perfection, and work from or back 

 to its newly-acquired shape, as the pressure upon it may be applied. 

 This law has been worked out with admirable ingenuity. Thus, a 

 simple piece of well-made metal tube is first partially flattened in all 

 its length, and coiled nearly to a circle. One end of it is stopped 

 up, while the other end is left open, to receive the pressure of steam or 

 water. To the end that is stopped a hand is fixed, which is so placed 

 as to show the variations in the position of the tube upon a dial mark- 

 ing the degrees of pressure. Here is a most perfect pressure-gauge, 

 of a simplicity hitherto beyond conception. A vacuum-gauge is made 

 of the same simple piece of mechanism, reversing the application of 

 pressure, and, consequently, the effect upon the tube. For instance, 

 as exhaustion takes place in the tube, so does its power of resisting the 

 pressure of the surrounding atmosphere, which acts upon it, vary, and 

 it consequently again coils under that pressure in regular ratio with 

 the variation of it, and is made to indicate the degree of vacuum in the 



