Penfield and Forbes — Chrysolite- Fay alite Group, etc. 129 



taut radial spokes attached at the inner face of the curl with 

 their central ends fastened to a fiber of silk. 



If, therefore, the conditions investigated at the end of 

 §§ 6, 10, be called to mind, a non-differential curl aneroid (§ 8, 

 case 3) less than a meter long, provided with a mirror for regis- 

 try, will give account of variations of atmospheric pressure of 

 a thousandth of a millimeter of the barometer, provided the 

 mounting is sufficiently free from tremor, and temperature be 

 kept constant to a few thousandths of a degree during the inter- 

 val of observation. 



These conditions will be much less severe if the parts of the 

 counter-twisted system are especially chosen (as stated in § 8, 

 case 2) and twisted with reference to viscosity, rigidity and 

 temperature. Indeed the chief result of the present paper is 

 the exhibition of the properties of the counter-twisted system. 

 A continuous mechanism has been brought forward which not 

 only minimizes the hurtful effects of viscosity and of the ther- 

 mal changes both of viscosity and of rigidity, but which 

 accomplishes these desirable results in such a way as to remark- 

 ably increase the sensitiveness of the instrument. 

 Wilson Physical Laboratory, 



Brown University, Providence, R. I, 



Art. XYI. — Fayalite from Rockport, Mass., and on the 

 Optical Properties of the Chrysolite- Fayalite Group and of 

 Monticellite ; by S. L. Penfield and E. H. Forbes. 



In the fall of 1890, Mr. J. H. Sears of the Peabody Academy 

 of Science, Salem, Mass., while visiting the Rockport Granite 

 Quarry, found a crystalline mass of a dark colored mineral, 

 which proved on examination to be fayalite. The occurrence 

 of this mineral is so unusual that it was considered worthy of 

 a special investigation and was sent for that purpose to the 

 Mineralogical Laboratory of the Sheffield Scientific School. 



The material offered an excellent opportunity for an investi- 

 gation of the optical properties of fayalite, which had never 

 been determined, and the results were of such a nature that it 

 seemed best to extend the study to the different members of 

 the chrysolite group, in order to determine the effect upon the 

 optical properties due to the mutual replacement of iron and 

 magnesium. 



Fayalite. ' 



The material from Rockport was a crystalline mass, found 

 at a depth of 60 feet, near the base of a large boss, or vein of 



Am. Jour. Sci.— Fourth Series, Vol. I, No. 2.— February, 1896. 

 9 



