ON INSTRUMENTS FROM ITAL Y. 129 



specific gravity varied with the temperature, it was thought that one 

 experiment would suffice to give both the data necessary in order to 

 determine it with precision. In this photograph you can see one of 

 the methods employed viz., by fixing a thermometer in a hydro- 

 stamm ; but other ones of the Accademia del Cimento exist, which are 

 true thermometers, having, in addition, the graduation of the hydro- 

 stamm. 



The Grand Duke also occupied himself in perfecting a certain 

 pendulum, in order to obtain equable intervals of time of greater or 

 less duration ; and from the figure which you see here in the t( Libra 

 de' Saggi di Naturali Esperienze," you can understand how for the 

 oscillating ball always to remain in the same plane, it was suspended 

 by two threads, which thus formed the two sides of an isosceles tri- 

 angle, and of which it was possible to alter the length, at pleasure, by 

 means of a convenient pincers. This pendulum was also used in the 

 experiments made upon the velocity of propagation of light and 

 sound. Indeed concerning the latter, which were afterwards repeated 

 by the Accademia del Cimento, I cannot but say a few words, as in 

 the usual works on physical science mention is only made of the 

 experiments carried out by the French academicians, while those 

 made so many years before, upon the same plan and with the same 

 results, are omitted. 



Here is an extract from one of Viviani's letters, in which he gives 

 an account of these experiments. After having related certain dis- 

 cussions upon sound which had taken place with Signer Rinaldini and 

 Signer Borelli, and had spoken of some bomb-shots that had been 

 fired from the Petraja, he having been with the Grand Duke when 

 they were determining its distance from Florence, he continues : 



" His Highness asked me the following questions Which of the two 

 sounds, the greater or the less, reached the ear in the shortest time ? 

 To this I answered that both would reach exactly in the same time. 

 Secondly, of what impediment the wind could be to the propagation 

 of sound ? I answered : None. He then proceeded with his inquiries, 

 and asked me what difference of time I thought there would be in the 

 rate of the sound, between the discharge of the piece with the mouth 

 turned towards the ear of the observer, or turned up perpendicularly, 

 or turned the other way ? To which I answered immediately, although 



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