NATURAL 1'HlLOSOi'HY. 191 



same precise principle which governs the one, controls all ; this, 

 therefore, must be considered a suilieicnt j)roof't!iat the only centre of 

 the moon's motion is at the centre of the planet, as are the rings to 

 Saturn, and, like them, our moon docs not revolve upon its own axis, 

 and until cosmical laws are changed cannot do so. Communicated by 

 ( 'Itarles E. Townsend, Esq. 



SOUND AS A TIME MEASURE, 



In a recent French work, entitled Trait6 des Mecanismes, by M. 

 Goupilliere, we find described a curious and ingenious method of meas- 

 uring time, which gives thousandth parts of a second. The descrip- 

 tion is substantially as follows : Suppose it were required to measure 

 the exact time of the descent of the hammer of a gun-lock on the nip- 

 ple. The motion is so rapid that the most delicate stop-watch is at 

 fault. A needle might be fixed to the hammer, so as in descending to 

 mark a curve on a blackened metal plate ; but still the time would be 

 an unknown quantity. It may, however, be measured by means of a 

 tuning-fork, also provided with a marking-needle ; then, while the for- 

 mer one marks the curve described by the hammer, the second needle 

 will mark the vibrations of the fork ; and as we know that they are 

 isochronous, each of the small insinuosities thus obtained on the black- 

 ened plate will represent a fraction of time, and show how many such 

 fractions elapsed before the fall of the hammer. To give an idea of 

 the degree of precision which may be obtained by this process, let us 

 suppose the normal French tuning-fork, which will perform 896 vibra- 

 tions in a second ; then the duration of each vibration will be -g^g- of a 

 second ; and as the greatest error that can be committed cannot ex- 

 ceed half a vibration, the measurement will be exact to yyW ^ a 

 second. 



ON A NEW MODE OF DETERMINING THE VELOCITY OF SOUND. 



The following new mode of determining the velocity of sound was 

 proposed, by Dr. J. Stevelly, at the last meeting of the British 

 Association. 



Suppose a piece of clock-work prepared, for instance, to strike 

 .single strokes upon a bell each time the detent is set free ; the detent 

 to be under the control of an electro-magnet, which is instantly 

 set in action by an observer, at a measured distance from the bell or 

 other origin of sound, depressing a key, and thus completing a gal- 

 vanic circuit. The observer, being furnished with a chronometer, 

 depresses the key ; the instant he hears the stroke of the bell he again 

 depresses it; hears a second sound, and so goes on for 100 or 1,000 

 times, carefully noting by the chronometer the instant at which he 

 hears the last sound of the series. A trained observer would not make 

 a probable error of one-tenth of a second in noting the whole time 

 occupied by the whole series ; and to avoid all chance of miscounting 

 the number of sounds in the series, the clock may be readily made to 

 keep count of the number of strokes it makes. The whole time occu- 

 pied by the entire series is made up of the following portions : 1. 

 The time consumed in the mechanical work of the clock in producing 

 the stroke, and of the key, from the instant the observer touches it 

 until it has completed the circuit. 2. The personal equation of 



