120 Description of an Eriometer. L Aio. 



length; th^ ends next the skin are almost always considerably 

 finer, and the outer ends generally coarser, than the rest ; but 

 this difference is greater in some kinds of sheep than in others, 

 and as far as I have observed, it is less in the Merinos and their 

 crosses than in other sheep : there is also far less difference in 

 the different parts of the same fleece in these breeds than in 

 others ; still however this difference is very observable, although 

 it is probable that some part of the sheep might be found which 

 in all cases might fairly be considered as affording nearly the 

 average of the whole fleece; and I imagine that the part of the 

 back about the loins is the most likely to be possessed of this 

 property ; so that the middle of the fibres of this part of the 

 fleece might be assumed, in the finer kinds of wool, as affording 

 a fair measure for the whole. 



III. Scale of the Eriometer. 



The theory which suggested to me the construction of the 

 eriometer requires some corrections in its immediate applica- 

 tion, which depend on circumstances not completely under- 

 stood : at present, therefore, I shall only employ, for the deter- 

 mination of the true value of the numbers of its scale, an expe- 

 rimental comparison of its indications with some microscopical 

 measurements, which Dr. Wollaston has been so good as to 

 perform for me, with an admirably accurate micrometer of his 

 own invention. 



The dust or seed of the lycoperdon bovista he finds to be -5-^-5- 

 of an inch in diameter : this substance gives very distinctly 3*5 

 on the scale of the eriometer; and 3*5 x 8500 = 29/50. The 

 globules of the blood measured . 4 - g ' 00 ; and immediately after- 

 wards, when examined in the same state by the eriometer, indi- 

 cated about 6^ ; and 6*5 x 4900 = 31 850. A wire of platina, 

 obtained bv a very ingenious method, peculiar to Dr. Wollaston, 

 measured 2 J 00 ; and when coiled up, gave n. 9 of the eriometer; 

 and 9 x 3200 = 28800. The mean of a considerable number of 

 comparative observations on fibres of wool, between n. 20 and 

 30, afforded also 28S00 for a product. 



A mean of these experiments gives very nearly -j-hro f° r tne 

 unit of the scale of the eriometer. Some former investigations 

 had led me to attribute to this unit a value somewhat smaller, 

 especially for the lowest numbers; and I had obtained a formula, 

 and made a table, for ascertaining the true dimensions of any 

 substance measured by the instrument, according to the result of 

 these investigations : but since my later experiments seem to have 

 superseded the mode of calculation which I had adopted, I think 

 it unnecessary to insert the table. 



Having sufficiently ascertained the true value of the indica- 

 tions of the eriometrical table, 1 shall now enumerate the mea- 



