152 ANNUAL OF SCIENTIFIC DISCO VERY. 



A NEW THERMOGRAPH. 



M. Marcy has addressed to the French Academy the folio-wing 

 description of an instrument for marking small variations of tempera- 

 ture : 1. The first part of this thermograph consists of a copper tube 

 a meter in length, the interior diameter of which is capillary, not being 

 more than \ of a millimeter. It is open at one end, and soldered to 

 a hollow copper ball at the other end. 2. The second part of the 

 apparatus consists of a wheel resting upon knife-edges, like those of 

 a pair of scales, whereby a very delicate oscillation may be imparted 

 to it. The axle of the wheel carries a long vertical needle, marking 

 the degrees on a circular scale. To the circumference of the wheel is 

 fixed a glass tube six millimeters in diameter, and bent in conformity 

 to the curvature of the wheel, and so situated that the middle of the 

 tube lies vertically underneath the needle when the wheel is at rest. 

 One of its extremities is hermetically closed, while the other is open. 

 Now, if a little mercury be poured into this tube it will settle at the 

 lowest point, and the interior of the tube will thus be divided into two 

 chambers, one closed and with air confined in it, the other open. 

 3. Now introduce the copper tube into the glass one, giving it of 

 course the same curvature, and so that its extremity may pass through 

 the mercury, thus establishing a communication between the hollow 

 copper ball and the confined chambers, and the apparatus, with a few 

 accessory appliances, will be complete. The end of the copper tube 

 dipping into the mercury should be varnished to prevent its being 

 attacked by the latter metal ; or better still, the end might be made of 

 platinum. 4. To use this apparatus, put your hand to the copper 

 ball ; the warmth thus imparted to it will dilate the air it contains, and 

 drive part of it into the confined chamber ; the mercury will therefore 

 recede, and thereby make the wheel turn round its center of gravity ; 

 the very small arc thus described will be revealed by the needle, the 

 difference of its present position with its previous one when at rest. 

 If, on the contrary, the copper ball be cooled, by water, for instance, 

 the air inside will be contracted, a portion of the air of the confined 

 chamber will rush in, and the mercury will be driven forward, the 

 needle turning in the inverse direction. By means of this experiment 

 very delicate physiological experiments on animal heat may be con- 

 ducted. 



HEAT AND FORCE. 



Whenever friction is overcome, heat is produced ; and the amount 

 of heat so produced is the exact measure of the force extended in 

 overcoming the friction. Prof. Tyndall, speaking upon the subject of 

 "Heat considered as a Mode of Motion," says : "We usually put oil 

 upon the surface of a hone ; we grease the saw, and are careful to 

 lubricate the axles of our railway carriages. What are we really 

 doing in these cases ? Let us get general notions first ; we shall come 

 to particulars afterwards. It is the object of the railway engineer to 

 urge his train boldly from one place to another. lie wishes to apply 

 the force of his steam or his furnace, which gives tension to the steam 

 to this particular purpose. It is not his interest to allow any portion 

 of that force to be converted into another form of force which would 



