i72 m\mi onGANUM. [book It. 



further back. Let an accurate experiment, tlierefore, be made 

 with a stick, or something of the kind, put into the flame, in 

 order to see whether it be not sooner burnt at the sides than in 

 the middle of it.« 



37. There are many degrees in the susceptibility of heat. 

 And, first, it must be observed how much a low gentle heat 

 changes and partially warms even the bodies least susceptible of 

 it. For even the heat of the hand imparts a little warmth to a 

 ball of lead or other metal held a short time in it : so easily is 

 heat transmitted and excited, without any apparent change in the 

 body. 



38. Of all bodies that we are acquainted with, air admits and 

 loses heat the most readily, which is admirably seen in weather- 

 glasses, whose construction is as follows. Take a glass with 

 a hollow belly, and a thin and long neck ; turn it upside down, 

 and place it with its mouth downwards into another glass vessel 

 containing water ; the end of the tube touching the bottom of 

 the vessel, and the tube itself leaning a little on the edge, so as 

 to be fixed upright. In order to do this more readily, let a little 

 wax be applied to the edge, not however so as to block up the 

 orifice, lest by preventing the air from escaping, the motion, 

 which we shall presently speak of, and which is very gentle and 

 delicate, should be impeded. 



Before the first glass be inserted in the other, its upper part 

 (the belly) should be warmed at the fire. Then upon placing it 

 as we have described, the air (which was dilated by the heat), 

 after a sufficient time has been allowed for it to lose tlie addi- 

 tional temperature, will restore and contract itself to the same 

 dimensions as that of the external or common atmosphere at the 

 moment of immersion, and the water will be attracted upwards 

 in the tube to a proportionate extent. A long narrow slip of 

 paper should be attached to the tube, divided into as many 

 degrees as you please. You will then perceive, as the Aveather 

 grows warmer or colder, that the air contracts itself into a 

 narrower space in cold weather and dilates in the warm, which 

 will be exhibited by the rising of the watei as the air contracts 

 itself, and its depression as the air dilates. The sensibility of 

 the air with regard to heat or cold is so delicate and exquisite, 

 that it far exceeds the human touch, so that a ray of sunshine, 

 the heat of the breath, and much more, that of the hand placed 

 on the top of the tube, immediately causes an evident depression 

 of the water. "We think, however, that the spirit of animals 



* If condensation were the cause of the greater heat. Bacon con- 

 cludes the centre of the flame would be the hotter part, and vice 

 versa. The fact is, neither of the causes assigned by Bacon is the 

 true one ; for the fire bums more quickly only, because the draught 

 of air is more rapid, the cold dense air pressing rapidly into the 

 heated room and towards the chimney. 



