CLAY WORKING INDUSTRIES. 85 



useful. Instances of this kind are not uncommon, but, after all is said 

 and done, the proof is by trial alone. 



There are certain aids to a proper trial of a clay which are beginning 

 to find use in clay works in the country. Among them are various 

 forms of pyrometers for measuring the heat employed in burning. An 

 instrument which is accurate and convenient and portable has long been 

 needed in the clay working industries. 



There are a large number of patented pyrometers in the market, none 

 of which have proved their usefulness to the clay-worker. Those de^ 

 pending on the expansion of a metallic bar by heat are not conveniently 

 portable and are easily deranged and the bar is constantly liable to give 

 out, as the temperatures employed in clay working are much higher than 

 the metallurgical processes for which these bar expansion pyrometers are 

 especially designed. 



A new pyrometer in the market depends on the expansion of a 

 column of air enclosed in a strong but thin metallic tube. This instrument 

 is claimed to be very accurate but is very expensive. One of the com- 

 paratively recent forms of heat measuring appliances is the I/unette Pyro- 

 metrique or pyrometric telescope invented by M. M. Mesure & Novel 

 and manufactured by E. Ducrete, of Paris. This instrument is an optical 

 one and depends on the use of two nicol prisms, one fixed and the other 

 capable of revolution on its axis, and separated from the other by a quartz 

 disc or plate. In looking at a radiant heated object with this instrument, 

 the light transmitted, when the two prisms are at a certain position with 

 respect to one another, is almost colorless. When the movable prismas 

 revolved on its axis by means of a graduated hand wheel, the color 

 changes through the usual range of colors produced in the polariscope. 

 There is one point in this array which presents a reasonably sharp- tint, 

 and that is when the green colors fade away to be replaced by red. 'In 

 using the instrument, the operator sets the prisms in the colorless rela- 

 tion, as indicated by the zero mark on the hand wheel, and fixing his gaze 

 on the hot object to be measured proceeds to turn the hand wheel when 

 the colors run through the green series and into the pink or reds. The 

 transition point between the two tints is selected as the reading 'point, 

 arid when this has been determined by rotations backward and forward, 

 the number of degrees of rotations are read off from the graduated hand 

 wheel. This figure can then be turned into degrees, Centrigrade or Fahr- 

 enheit, if desired, by comparison with a scale of known heats' and read- 

 ings taken from them by the Lunette. 



This instrument has been in use by the Survey during the visits paid 

 to clay working establishments in all quarters of the state. A very large 

 number of readings were taken and the instrument was found to be con- 

 sistent with itself on all occasions. The heat of a certain fire being read 

 by one man several times would give a series of closely agreeing read- 

 ings, and different parties, including some who had never seen such an in- 



