for the forces which produce the deviations 35° and 16°, the first (46·7) will be found in the table, but the second, being under 20°, will have the same value as the arc; that is to say, 16. When we want to find the forces which correspond to fractions of a degree, we have only to ascertain the proportional part of the degree in question; for, in the interval between one degree and another, the curve visibly coincides with the tangent. If, for example, we wish to know the force that corresponds to the deviation 31°·7, it will be sufficient to take at first the difference between 37·4 and 39·6 (the intensities of the forces belonging to 31° and 32°); this difference being 2·2, we shall find the value (x) of the force corresponding to seven tenths of the degree contained between 30° and 32° by this proportion,
1° : 0°·7 :: 2·2 : x ═ 1·5.
Adding this to the number 37°·4, which represents the force corresponding to 31°, we shall have 38·9 as the value sought.
Of the Polish, the Thickness, and the Nature of the Screens.
The suggestions which we have offered as to the manner of measuring the quantity of caloric instantaneously transmitted by diaphanous bodies, and as to the precautions to be taken during the experiments, leave us scarcely anything more to say on this subject. Nevertheless it may not be amiss to mention some particulars relative to the construction of the apparatus before we proceed to the exposition of the results.
The pile employed in these researches is of the form of a quadrangular prism; its two ends are plane surfaces, each measuring 4·24 centimetres; it consists of 27 pairs and a half, or 5 elements of bismuth and antimony, 32 millimetres long, 2·5 broad, and 1 in thickness. It was not without considerable difficulty that we succeeded in combining and soldering together these minute bars. The facility with which liquid antimony oxidizes, the difference between its fusibility and that of the bismuth, and the extreme fragility of the two metals, presented so many obstacles, that it cost many an effort to overcome them. But a pile of very small dimensions was indispensable in the investigation of the laws of immediate transmission through rare liquids and crystallized solids. This was, therefore, to be obtained, or the experiments to be abandoned. By this conviction we have been induced to persevere in spite of repeated disappointments, and by redoubling our patience have at last succeeded.
The electric pile is passed into a ring formed of a thin square flake of copper internally lined with pasteboard and having a screw which serves to fix it on the stand, so that the axis naturally takes that horizontal position which it is to keep during the greatest part of the experiments. To each side of the ring there is fitted a tube of six cen-
