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here, in future, to have the movements numbered in correspondence with this plexi-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

Adagio Andante Allegro Presto

52 &quot;|

60 I times vibrates &amp;lt;( 70 &amp;gt;in a

95 135J

minute.

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

For a harpsichord, the following occurs to me.

Adagio, 2 I* 1 tne wa ^ f vour chamber, over the in-

Andante, 8.1? N. strunient, drive five little brads, as, 1, 2, 3,

Presto 5 A, 4 ^ * n ^ ie fN w i n g 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, &tc. ; 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 dis covered that lenses of certain natural chrystals 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