144 ANNUAL OF SCIENTIFIC DISCOVERT. 



JOULE'S NEW SENSITIVE THERMOMETER. 



At a recent meeting of the Manchester Philosophical Society, Dr. 

 Joule exhibited an exquisitely sensitive air thermometer, capable of 

 being affected by the y-gVo" f a centigrade degree of heat. The con- 

 struction is thus described : A glass vessel in the shape of a tube, two 

 feet long by four inches in diameter, is divided longitudinally by a 

 blackened pasteboard diaphragm, leaving spaces at the top and bottom, 

 each little over an inch. In the top space, a piece of magnetized sew- 

 ing needle, furnished with a glass index, is suspended by a single fila- 

 ment of silk. It is evident that the arrangement is similar to that of a 

 bratticed coal-pit shaft, and that the slightest excess of temperature on 

 one side over that on the other must occasion a circulation of air, which 

 will ascend on the heated side, and, after passing across the fine glass 

 index, descend on the other side. It is also evident that the sensibility 

 of the instrument may be increased to any required extent, by dimin- 

 ishing the directive force of the magnetic needle. I purpose to make 

 several improvements in my present instrument ; but in its present con- 

 dition, the heat radiated by a small pan, containing a pint of water 

 heated 30, is quite perceptible at a distance of three yards. A further 

 proof of the extreme sensibility of the instrument is obtained from the 

 fact that it is able to detect the beat radiated by the moon. A beam 

 of moonlight was admitted through a slit in the shutter. As the moon 

 (nearly full) travelled from left to right, the beam passed gradually 

 across the instrument, causing the index to be deflected several degrees, 

 first to the left and then to the right. The effect showed, according to 

 a very rough estimate, that the air in the instrument must have been 

 heated by the moon's rays a few ten-thousandths of a degree, or by a 

 quantity, no doubt the equivalent of the light absorbed by the black- 

 ened surface, on which the rays fell. 



CHANGE OF FORM IN METALS BY IRREGULAR COOLING. 



Colonel H. Clerk has communicated to the Royal Society some curi- 

 ous experiments on this subject. It appears that a wheel had to be 

 shod with a hoop-tire, which was required to have a bevel of about 

 f ths of an inch, and one of the workmen suggested that this could be 

 accomplished by heating the tire red-hot, and immersing one-half its 

 depth in cold water. This was done, with the predicted result ; the part 

 out of the water being reduced in diameter. A series of experiments 

 followed, with similarity of action, the cylinders always exhibiting a 

 contraction above the water-line, followed, if they were sufficiently high 

 out of the water, by an expansion corresponding to that below the fluid. 

 The explanation given is, that the parts under the water cooled quick- 

 ly, and those above it slowly. If no cohesion had united the two parts, 

 both would have obtained the same diameter, one first, and the other 

 afterwards ; but as the cohesive power of cast-iron, or other metal, is 

 great, the under part tends to pull in the upper, and the upper to pull 

 out the under. In this contest, the cooler metal, being the stronger, 

 prevails, and so the upper part gets pulled in, a little above the water- 

 line, while still hot. But it has still to contract in cooling, and this it 

 will do to the full extent due to its temperature, except so far as it may 

 be prevented by its connection \vilh the rest. Proceedings of the 

 Royal Society. 



