chronometer. I need
not tell you that the numbers between two movements, as between 22 and
36, give the quicker or slower degrees of the movements, such as the
quick _andante_, or moderate _allegro_. The instrument is useful, but
still it may be greatly simplified. I got him to make me one, and having
fixed a pendulum vibrating seconds, I tried by that the vibrations of
his pendulum, according to the several movements. I find the pendulum
regulated to Largo

[Illustration: The Plexi-Chronometer, page391]

Every one, therefore, may make a chronometer adapted to his instrument.

For a harpsichord, the following occurs to me:

In the wall of your chamber, over the instrument, drive five little
brads, as, 1, 2, 3, 4, 5, in the following manner. Take a string with
a bob to it, of such length, as, that hung on No. 1, it shall vibrate
fifty-two times in a minute. Then proceed by trial to drive No. 2, at
such a distance, that drawing the loop of the string to that, the part
remaining between 1 and the bob, shall vibrate sixty times in a minute.
Fix the third for seventy vibrations, &c.; the cord always hanging over
No. 1, as the centre of vibration. A person playing on the violin may
fix this on his music-stand. A pendulum thrown into vibration will
continue in motion long enough to give you the time of your piece. I
have been thus particular, on the supposition that you would fix one of
these simple things for yourself.

You have heard often of the metal called platina, to be found only in
South America. It is insusceptible of rust, as gold and silver are, none
of the acids affecting it, excepting the _aqua regia_. It also admits
of as perfect a polish as the metal hitherto used for the specula of
telescopes. These two properties had suggested to the Spaniards the
substitution of it for that use. But the mines being closed up by the
government, it is difficult to get the metal. The experiment has been
lately tried here by the Abbe Rochon (whom I formerly mentioned to
Mr. Rittenhouse, as having discovered that lenses of certain natural
crystals have two different and uncombined magnifying powers), and he
thinks the polish as high as that of the metal heretofore used, and
that it will never be injured by the air, a touch of the finger, &c. I
examined it in a dull day, which did not admit a fair judgment of the
strength of its reflection.

Good qualities are sometimes misfortunes. I will prove it from your
own experience.