270 SCIENTIFIC AMUSEMENTS. 



and so set up a current of air. The ascent of smoke in a chimney 

 is due to the same cause. 



When a current of air strikes upon a surface inclined to its 

 direction in a certain way, pressure is produced on the surface, 

 tending to make it move ; the turning round of the sails of a wind- 

 mill, and of several children's toys which revolve when a gentle 

 wind blows upon them, is thus brought about. In the same way, 

 if the top of a chimney be fitted with a revolving head where 

 the issuing smoke and hot air pass out through orifices radiating 

 out from the centre and bounded by flat surfaces fixed obliquely 

 to the direction in which the current issues, a continuous rota- 

 tion of the head is set up ; by connecting the revolving head with 

 a spit in front of the fire an automatic turning round of the joint 

 to be roasted is brought about. 



Trace a spiral line on a piece of card or thin sheet metal and 

 then cut through the card along the line with a sharp knife, &c.; 

 pull the centre of the spiral outwards so that the whole presents a 

 conical appearance, the surface of the card winding round and 

 round screw-wise, but each successive turn being larger in diameter. 

 Balance the central point of the " flat spiral " thus formed on a 

 needle stuck point upwards in a cork in the neck of a wine bottle, 

 or held by any convenient form of stand ; on bringing a candle 

 underneath the spiral, the ascending current of hot air will impinge 

 on the surface of the spiral, producing the same effect as wind on 

 the vanes of a windmill, setting the spiral in rotation. Toys based 

 on this principle are often met with, continuous revolution being 

 set up by fixing the toy near a fire over the edge of a mantel- 

 piece, with a candle underneath, or in some other situation where 

 an ascending current of warm air can play upon the vanes. 



Expt. 319. Differential Air Thermometer. Provide two small 

 flasks of equal size and two tightly-fitting india-rubber perforated 

 corks, with a piece of quill tubing about 2 feet long fitting air-tight 

 into the corks when fixed in the mouths of the flasks. Bend the 

 glass tube to a double right angle so as to form a large U, so that 

 each limb may be about 8 inches long and the two limbs about 8 

 inches apart ; when cool pour into the tube enough water (tinted 

 with indigo or ink, &c.) to fill the limbs about 4 inches high with 

 fluid. Now fix on the flasks at the two ends of the tube, so as to 

 form a double-bulbed air thermometer. If one bulb be warmed 

 by the hand or a flame, &c., the air therein will expand, and will 

 press the column of coloured fluid in the U tube down on the 

 warmed side and up on the other; so that any difference of 

 temperature between the bulbs is indicated by a variation in the 

 relative levels of the surface of the fluid in the two limbs. Fig. 



