OF RADIANT HEAT THROUGH DIFFERENT BODIES. 15 



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 M'ish to know the force tiiat coi'- 

 responds to the deviation 31°*7, it will be sufficient to take at first the 

 difference between 37"4 and 39"(i (the intensities of the forces belong- 

 ing 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 : a; = 1-5. 

 Adding this to the number 37"4', which represents the force correspond- 

 ing to 31°, we shall have 38*9 as the value sought. 



Of the Polish, (he 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 quadrangu- 

 lar prism ; its two ends are plane surfaces, each measuring 4"24" centi- 

 metres ; 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 

 verj' small dimensions was indispensable in the investigation of the laws 

 of immediate transmission through rare liquids and ciystallized solids. 

 This Avas, therefore, to be obtained, or the experiments to be aban- 

 doned. 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 hori- 

 zontal position which it is to keep during the greatest part of the ex- 

 l)eriment9. To each side of the I'ing there is fitted a tube of six cen- 



