296 



connect the wires at their extremities, so as to avoid as much 

 as possible the elasticity which is being constantly multiplied 

 by arranging the rods at unequal distances from the centre in 

 a straight line each side of the centre. 



The whole number of rods or wires in the bundle is eleven, 

 six of these (in three pairs) are iron, and on these the force 

 of compression is exerted. The force of extension is applied 

 to four brass wires (two pairs), arranged alternately with the 

 iron wires around the centre, and also to a single brass wire 

 in the centre (which single brass wire is the equivalent of a 

 pair). 



It will be seen that the above arrangement makes the first 

 pair of iron rods form the base of the Thermometer, and the 

 last (single) brass wire the terminus. 



The base of the bundle of rods is secured against a proper 

 foundation in a pendant position : on this foundation is placed 

 the support for the fulcrum of a very strong lever, bearing on 

 knife edges. The short arm of the lever is connected with 

 the central brass wire of the bundle, and multiplies the differ- 

 ence of the rates of expansion of the two metals composing 

 it eight times ; a second lever still further repeats this five 

 times. The long arm of the second lever, by means of a 

 chain (made of flat links and rivets), communicates movements 

 to a pulley resting on friction wheels, which pulley is made in 

 two parts, one of which is eight times larger than the other 

 (allowance being made for the semi-diameter of the chain on 

 the smaller portion of the pulley). 



A slender silk cord runs over the larger part of the pulley 

 and sustains a delicate weight, which is guided between two 

 slender parallel rods (brass wire No. 32 wire measure), which 

 parallel rods are kept in tension by means of springs. 



The above comprehends the essential features of the Ther- 

 mometer. All the parts should be constructed with as much 

 nicety of adjustment as a fine watch, and with the exception 

 of the two metals in the rods, and perhaps the knife edges 

 and shafts or pivots of the friction wheels, the whole work 

 should be of brass, of a uniform quality and temper as near 



(Continued on page 316.) 



