VOL. LXXXII.] PHILOSOPHICAL TRANSACTIONS. 141 



jugglers in the country: this meal 



covers the hand, and descending along The bulb of the thermometer surrounded by 256 



. grains of semen l}xopodii. 



with it to the bottom of the basin, de- 



fends it from the water. This sub- 

 stance has the appearance of an ex- 

 ceeding line, light, and very moveable 

 yellow powder; and it is very inflam- 

 mable; so much so, that being blown 

 out of a quill into the flame of a 

 candle, it flashes like gunpowder, and 

 it is made use of in this manner 

 in our theatres for imitating light- 

 ning- 



The next question which arises is, how air can be prevented from conducting 



heat? and this necessarily involves another, which is, how does air conduct 

 heat? 



If air conducted heat, as it is probable that the metals and water, and all other 

 solid bodies and unelastic fluids conduct it, that is to say, if, its particles remain- 

 ing in their places, the heat passed from one particle to another, through the 

 whole mass, as there is no reason to suppose that the propagation of heat is 

 necessarily in right lines, I cannot conceive how the interposition of so small a 

 quantity of any solid body as -^ jiart of the volume of the air, could have ef- 

 fected so remarkable a diminution of the conducting power of the air, as ap- 

 peared in the experiment (N° 4) with raw silk above mentioned. 



If air and water conducted heat in the same manner, it is more than probable 

 that their conducting powers might be impaired by the same means; but when I 

 made the experiment with water, by filling the glass globe, in the centre of 

 which the bulb of the thermometer was suspended, with that fluid, and afterwards 

 varied the experiment, by adding \6 grains of raw silk to the water, I did not 

 find that the conducting power of the water was sensibly impaired by the presence 

 of the silk. But we have just seen that the same silk, mixed with an equal volume 

 of air, diminished its conducting power in a very remarkable degree ; consequently, 

 there is great reason to conclude that water and air conduct heat in a different 

 manner. 



After various remarks on the uses and effects of air and water, &c. in con- 

 fining or conducting heat, the author concludes with the following reflections: 

 The ocean may be considered as the great reservoir and equalizer of heat; and 

 its benign influences in preserving a proper temperature in the atmosphere 

 operate in all seasons and in all climates. The parching winds from the land 



