Appendix Ii.

CONTAINING A LETTER FROM DR. YOUNG TO COUNT RUMFORD, WHEN THE OFFER OF THE PROFESSORSHIP AT THE ROYAL INSTITUTION WAS MADE TO HIM; AND TWO LETTERS TO SIR JOSEPH BANKS ON THE INVENTION OF A MICROMETER TO BE USED FOR MEASURING WOOL. THE ORIGINALS OF THESE LETTERS ARE AMONGST THE PAPERS OF SIR JOSEPH BANKS.

DR. YOUNG TO COUNT RUMFORD.

Welbeck Street, Thursday, July 9, 1801.

SIR,--I have received your obliging letter, and beg leave to return you and the managers thanks for the honour you do me.

I am willing to undertake the various charges which you have the goodness to detail, and I flatter myself that you will have no reason to complain of any want of zeal on my part in the service of the Royal Institution.

But I confess I think it would be in some measure degrading both to me and to the Institution that the salary, which appears to me to have been no more than moderate before, should now be reduced one-fourth, at the same time that the labour and responsibility of the employment are rather increased than lessened. For, whatever might have been expected of the late professor respecting the Journals and the superintendence of the house, I do not apprehend that any specific stipulation was made on the subject; and, as I am determined to devote a greater share of attention to the Institution than he ever appears to have done, I do not see that my education and opportunities of literary acquirement have been such as to render me less worthy than he was of a salary which, when compared with the emoluments of other situations of a similar nature, is by no means exorbitant.

It would not be my wish, and the duties of the professorship would certainly render it impossible for me, to attempt any extent of medical practice; but I should be sorry to bind myself to reject the little that might accidentally fall in my way, I do not mention this as a matter of any consequence, but to avoid having it understood, from the conversation I had with you, that I should be obliged to refuse my advice to a friend who might consult me.

As to the Journals, I should not much object to engage that a sheet or more should be ready for publication every week; but I conceive that it would give them additional importance if it were left to the direction of the professor, with the approbation of the committee, with proper notice, to publish a number at the end of a fortnight, instead of a week, whenever there might appear to be a real deficiency of matter to fill it. And I think I should want little or no assistance, either in translating or in transcribing, except what Mr. Davy might have the goodness to give me.

I hope that, when you have reconsidered what I have stated, you will not much differ from me in opinion, and that you will favour me with a further communication of your sentiments on the subject.

I am, Sir, your obliged and obedient humble Servant,

THOMAS YOUNG.

Count Rumford, Royal Institution.

DR. THOMAS YOUNG TO SIR JOSEPH BANKS.

Worthing, September 10, 1810.

DEAR SIR,--Observing from the papers that you have been interesting yourself respecting the arrangement of a micrometer for the purpose of measuring the diameter of the fibres of different kinds of wool, I beg leave to trouble you with the description of a very simple instrument which I invented some time ago for a similar purpose, and which I propose to call an agricultural micrometer. I should imagine it to be sufficiently accurate for all practical purposes, and the great facility and cheapness of its construction may perhaps render it useful to a class of persons who would object to the expense of providing themselves with a more complicated apparatus. If it appear to you in the same point of view, you will be pleased to make any use that you may think proper of this communication. When we look at a distant candle through a lock of wool it appears surrounded by rings of colours, and these rings are invariably so much the larger as the wool is the finer. The cause of these appearances I have endeavoured to explain in my lectures and elsewhere; but for the present purpose the principal object is to ascertain their comparative magnitude. In order to perform this I take a card blackened on one side, describe on it two concentric circles, the outer exactly one inch in diameter, the inner 11/20, make at the centre a hole about 1/20 of an inch in diameter, and pierce the card at the circumference of the inner circle, in 10 or 12 points at equal distances, with a small pin, and at that of the larger in 7 or 8 points, at unequal distances, with a large one. I then take a small rod of wood about a yard long and divide it into half-inches, numbering them from one end, at which I fix two or three pins side by side and wind round it a piece of wire--for instance a common knitting needle--in such a form as to hold the card upright between its ends and to slide on the rod. This is the whole apparatus. It is to be used by candle-light in this manner: Attach to the fixed pins a small lock of the wool to be examined, containing 20 or 30 single fibres, and look through them and through the central hole at a candle not very remote from the card, the blackened side of the card being turned towards the eye. The hole will appear to be surrounded by a bright surface, reddish at the margin, by a dark circle or ring, and again by a brighter ring, bluish-green within and red without. If the colours are not seen distinctly, we may conclude that the wool is mixed and not perfectly fit for examination; but in this case we may generally form an estimate of its quality by means of the bright surface only, moving the card along the rod until the holes of the inner circle appear exactly at the extreme margin of that surface, where it is of a reddish hue, the place of the card as indicated by the scale showing the number which characterises the wool. It will also be most convenient to begin by producing this coincidence of the inner circle in other cases where the outer ring of colours is more distinctly seen, and then to adjust the card more accurately; so that the holes in the circumference of the outer circle may appear to coincide with the middle of the coloured ring at the common limit of the red and greenish portions. If the holes be seen in the red ring, the card must be brought nearer to the edge; if in the blue, it must be moved farther off, and the number of the scale must be observed as before. It will require very attentive observation, and perhaps some practice, to obtain always precisely the same number for the same substance, but by taking the mean of several trials we may be perfectly certain of coming within a unit of the true place of the slider, and perhaps even of avoiding any error at all.

I have not yet had an opportunity of examining any great variety of substances, but I send you the result of such observations as I have made, which will be sufficient to enable you to judge of the accuracy of the instrument.

Fibres of coarse wool from green baize, 52. Southdown, 35. Anglo-merino from a flock of Mr. Henty Tarring, 27. A lock taken from a Paular ewe by Mr. Sheppard, 25 (varying from 24 to 30). Another specimen from the same flock brought by another gentleman as a test of the instrument, 25. Cotton, mixed but about, 19. Vigonia from the Rev. P. Wood, very distinct 14½. Beaver from a hat, 11. Blood diluted with saliva and rubbed on glass, beautifully distinct 5. Milk diluted with water, very indistinct 3. I have sometimes thought of employing the instrument as a nosological test of the state of the blood, of pus, and of other animal fluids.

It is of little consequence to the farmer to know the actual dimensions indicated by these numbers, nor can I at present ascertain them with perfect accuracy. They express, however, very nearly the diameters of the fibres in the 45,000th of an inch. Thus Mr. Henty’s wool, standing at No. 27, must measure about 27/45000 or 1/1667 of an inch in diameter, and the globules of the blood reduced to spheres about 1/9000. Probably these results are a little too small, and especially the latter, but by a comparison of a few measurements, made by means of other micrometers with these numbers, it would be easy to form a correct table of their true value, and it may safely be asserted that this instrument will enable us in some cases to be secure of avoiding any error amounting to the hundred-thousandth of an inch, and almost in all of being far within one ten-thousandth of the truth, without the use of any microscope, simple or compound.

In order to render the instrument still more portable we may employ a piece of tape as a measure, fixing to one end of it a double piece of card with an aperture, and with some pins projecting from its edge for holding the wool either between its folds or on the pins, and to the other end a small weight, serving to draw the tape tight through a hole in the blackened card. I enclose you the whole apparatus arranged in this manner, together with a few fibres of the Vigonia wool, which exhibit the colours in great perfection, constantly giving 14½ as the characteristic number. The directions for the use of the instrument might be engraved and printed on the card if it were thought desirable.

I also take the liberty of forwarding to you a letter which I have just received from a French gentleman, who claims the protection of the Royal Society. You will best judge whether the case requires or admits any exertion of your well-known liberality and kindness.

Believe me, dear Sir, with the highest respect and esteem, your faithful and obedient Servant.

DR. THOMAS YOUNG TO SIR JOSEPH BANKS.

Worthing, October 6, 1810.

MY DEAR SIR,--I shall be most happy in assisting you to form a judgment from your own observation of the utility of my little instrument; but I cannot forbear to trouble you with a few specimens of wool, which I imagine will exhibit the appearances so obviously as at least to convince you of the perfect practicability of the method. You will observe, by merely looking through them at a candle, that there is a manifest difference in the size of the rings, and if the colours are not sufficiently conspicuous when the card is used, the central hole may be made a little larger with a bodkin; and a common pair of spectacles, such as you would use in mending a pen, will be amply sufficient for remedying the flatness of the eye.

I do not apprehend that the different magnitude of the different fibres of the same fleeces is any objection to the use of the instrument; on the contrary, it possesses the singular advantage of detecting at once the inequality where it exists, and of giving the mean dimensions of the whole at the same time where the difference is not too great. I have mixed together, for example, two small specimens, which measured separately 21 and 31. The mixture, though evidently irregular, gave the dimensions of about 24, varying from 23 to 27, according to the part of the lock which was to be examined, and this is surely a much greater inequality than can ever exist in any contiguous part of the same fleece. But, however this may be, the fact is that the circumstance does not actually destroy the validity of the indications of my micrometer, as I shall further exemplify to you by an account of my examinations of some specimens of the finest wool, with which I have been favoured by Mr. H. Sheppard, an ingenious manufacturer at Frome, Somersetshire. You are, perhaps, better acquainted than I am with the history of Mr. Western’s flock, which stands in so elevated a situation between the Saxon and the Spanish productions.

Specimens of Wool from Mr. Sheppard, as measured by the Agricultural Thermometer.

Grey beaver wool No. 11½ to 12 Angola about 14 Prime Vigonia 14½ Foreign coney 15 American rabbits 15 Yellow rabbits 15 Scotch hares 15 Siberian hares 15, 16 British coney 16 Finest seal about 18 Alpaca (a single long hair) 18½-20 Goats 19 Saxon 20 Peruvian black 21, 22 Mr. Western’s Southdown, ‘reckoned the finest in the kingdom’ 23½ Lioneza 24-29 Peruvian light brown 29 Peruvian dark brown 31 Dust of the puff-ball (Lycoperdon borista) rubbed on glass, very distinct, giving about 1/12000 inch diameter 3½