1813.] Description of an Eriometer. 119 



instrument may be rendered more portable, though somewhat 

 less accurate, by merely making the perforations in a blackened 

 card, furnished with a graduated piece of tape. An eye not 

 short-sighted will generally require the assistance of a lens, when 

 the instrument is made of the most convenient dimensions, 

 which I have found to be such as to have two circles of points, one 

 at -i and the other \ of an iuch in semidiameter, with their cor- 

 responding scales. The central perforations are about -^ and -^ 

 of an inch in diameter; the points 8 or 10 only in each' circle, 

 and as minute as possible. The light of the sun might also be 

 employed, by fixing the circle of points at the end of the tube 

 of a telescope: but it rather adds glare than distinctness to the 

 colours : nor have 1 been able to gain any thing by looking 

 through coloured glasses, or by using lights of different quali- 

 ties. Where the object consists of fibres which can be arranged 

 in parallel directions, a fine slit in the plate or card affords 

 brighter colours than a simple perforation, and the points 

 must in this case be arranged in lines parallel to the slit ; but if 

 care is not taken to stretch the fibres sufficiently, the employ- 

 ment of the slit in this manner will make them appear coarser 

 than they really are. The colours will still appear, even if there 

 be a considerable difference in the dimensions of the fibres or 

 particles, but they will be so much the less distinct as the differ- 

 ence is greater. In this case the measure indicated will be 

 intermediate between the extreme dimensions ; although most 

 commonly it will be seen somewhat below the true mean, the 

 colours exhibited by the finer fibres prevailing in some degree 

 over the rest. The latitude, however, which the eriometer 

 affords in the regularity of the substances measured by it, and 

 its collecting into one result the effect of many thousands of 

 particles, or of an endless variety of small differences in the 

 diameters of fibres, give it an unquestionable preference over 

 every kind of micrometer which measures a single interval only 

 at once, with respect to all applications to agriculture or manu- 

 factures; for in reality there is not a single fibre of wool among 

 the millions which constitute a fleece, that preserves an uniform 

 diameter throughout its length, and the difference is still greater 

 between the fibres which grow on different parts of the animal j 

 so that to take a single measurement, or even any practical 

 number of measurements, by the most accurate micrometer, in 

 the usual acceptation of the term, for a criterion of the quality 

 of a fleece, can tend only to the propagation of error or conjec- 

 ture in the semblance of the minutest accuracy. Even with the 

 eriometer, the difficulty of obtaining a fair average of the qua- 

 lity of a sample of wool is extremely great ; it is absolutely 

 necessary to preserve the fibres as much as possible in their natural 

 relative situation, and to examine them near the middle of their 



