1036 Transactions of ths American Institute. 



country^ of ingenious inventors a want properly announced is lialf 

 supplied. I wish some of our inventors would get up a material 

 wliicli would be unchanged by heat or cold, moisture or dryness. 

 Wood comes nearest to this of anything we know. All other bodies 

 are easily changed in size by the action of heat. I remember the 

 time when Whitworth, who was known at that time more as a 

 manufacturer of fine tools than an inventor of rifles, had one of his 

 micrometers in this city. One part of the instrument consisted of a 

 small piece of steel which passed between two surfaces and indicated 

 the distance between them. When this piece of steel was cool it 

 would drop in very freely, but the merest touch with the fingers 

 would expand it so that it would stick and no longer fall down. 



Dr. D. D. Parmelee made a diagram of the well-known walking 

 figures, which are propelled by means of alternate expansion and 

 contraction. He then called Mr. Stetson's attention to it as an 

 evidence that wood is not so unchangeable after all. 



Mr. J, K. Fisher. — This subject is of great importance in regard to 

 the scale used by draughtsmen. If the scale expands and contracts 

 we never can make two measurements alike. Wood scales are tolera- 

 bly constant, but ivory scales, though much finer, are nearly worthless. 



Dr. J. W. Richards. — It occurs to me that as wood is changeable 

 chiefly on account of its hygroscopic properties, the best way to get a 

 perfectly uncliangeable substance would be to use charcoal. Scales 

 made of charcoal could not alter. 



Mr. J. Phin. — Why go to the trouble of getting up a special mate- 

 rial for scales, when a scale, no matter how accurately it may be 

 made, nor of what unchangeable material it may be constructed, can 

 be used only to take measurements from the very changeable sub- 

 stance on which the drawing is made, viz.,i3aper? Would not a bet- 

 ter way be to lay the scale down on the drawing itself? Then what- 

 ever change occurred in the drawing would occur in the scale too, 

 and thus their relative proportions would remain the same. 



Mr. J. K. Fisher. — We have all seen these scales. They are used 

 by draughtsmen simply because they are cheap. A paper scale costs 

 a few cents, while a proper scale is quite ex])ensive. 



Mr. J. Phin. — I had no intention of alluding to the chart scale 

 mentioned by Mr. Fisher. I referred to accurately constructed scales 

 laid down by the draughtsman himself on the very paper on which 

 the drawing is made, so that whatever changes occur in the one may 

 occur in the other also. 



