NATURAL PHILOSOPHY. 253 







Greenwich, be chosen, to which all others should be referred. Let every such 

 clock throughout the world indicate Greenwich time on the inner or station- 

 ary circle of figures, but when the clock is set up at any station, let the outer 

 circle be moved round, and so set that while the hour hand shows Green- 

 wich time on the inner circle, it may show local solar time on the outer one, 

 and let messages be dated in both local and Greenwich time. The perfect 

 convenience of this plan was obvious. It reconciled the necessity of keeping 

 local time with the advantage of uniform time, and got rid of any trouble in 

 reducing the one into the other. The system might be rendered more work- 

 able still by abolishing the distinction of cast and west longitude, reckon- 

 ing all east or all west from degree to 3GO minutes, and by abolishing the 

 distinction of a. m. and p. m., reckoning time from midnight to midnight, up 

 to twenty-four o'clock. 



SELF-INDICATING BALANCE BAROMETER. 



M. Secchi, of Rome, has recently invented a new construction of barome- 

 ter, which possesses the advantages of not being liable to be broken, of giving 

 the readings exact, which Aneroids and others do not, and of recording those 

 readings by self-acting mechanism. M. Secchi says that all improvements 

 hitherto have been limited to the employment of large tubes to avoid the 

 evils of capillary attraction, and to the obtaining of greater exactitude in the 

 readings. All attempts to make the instrument graphic that is to say, self- 

 acting to record the different variations, and to make the indications more 

 minute have not yet been successful. The principles on which the new 

 barometer is constructed will be understood by the following statement : 

 Suppose the cuvette of a barometer to be placed on a table and the glass 

 tube so arranged as to admit of its being lifted by hand. The force that will 

 be required to lift the tube will be equal to the weight of mercury in the tube, 

 or in other words, to the amount of atmospheric pressure exercised on the 

 mercury of the instrument. We shall therefore be able to really weigh the 

 pressure of the atmosphere by attaching the barometer (the tube only) to 

 one end of a balance, and weights to the other ; for it is evident that at every 

 change in atmospheric pressure a corresponding increase or decrease in 

 weight will have to be made at the other end of the balance to maintain 

 equilibrium. To ascertain the value of absolute pressure on a unity of sur- 

 face, it w r ill be necessary to take into consideration the weight of the tub,. 1 , 

 and also of the weight of that portion thereof which is immersed in the mer- 

 cury contained in the cuvette, and especially the internal sectional area. 

 The knowledge of the latter, so far from being an obstacle, is a positive ad- 

 vantage in construction; for, by increasing the sectional area, the force that 

 actuates the instrument Avill also be increased, and will consequently permit 

 of more exact and more minute readings. If the sectional area be ten squares 

 centimcti'es, and the pressure varies by centimetres in height, the weight 

 to be placed at the other end of the balance will be that of nineteen cubical 

 centimetres of mercury, or one hundred and thirty -five grammes ; while, if 



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