TO  C. H. CAY, Esq.

                                                                         Glenlair, 5th January 1865.

     We are sorry to hear you cannot come and see us, but you seem better by your letter, and I hope you will be able    for your travels, and be better able for your work afterwards, and not take it too severely, and avoid merimnosity   and taking over too much thought, which greatly diminishes the efficiency of young teachers. We have been here  since 22d ult., and are in the process of dining the valley in appropriate batches. We have had very rough weather this week, which, combined with the dining, has prevented our usual airings. The ordinary outing is to the Brig of   Urr, Katherine on Charlie and I on Darling. Charlie has got a fine band on his forehead, with his name in blue and    white beads.

     The Manse of Corsock is now finished; it is near the river, not far from the deep pool where we used to bathe.

     I set Prof. W. Thomson a prop. which I had been working with for a long time. He sent me 18 pages of letter of    suggestions about it, none of which would work; but on Jan 3, in the railway from Largs, he got the way to it,   which is all right; so we are jolly, having stormed the citadel, when we only hoped to sap it by approximations.

The prop. was to draw a set of lines like this  so that the ultimate reticulations shall all be squares.   The solution is exact, but rather stiff. Now I have a  disc A hung by wire D, between two discs B, C, the interval being occupied by air, hydrogen, carbonic   acid, etc., the friction of which gradually brings A to rest. In order to calculate the thickdom or viscosity of the gas,
     I require to solve the problem above mentioned, which is now done, and I have the apparatus now ready to begin.    We are also intent on electrical measurements, and are getting up apparatus, and have made sets of wires of alloy   of platinum and silver, which are to be sent all abroad as standards of resistance. I have also a paper afloat, with    an electromagnetic theory of light, which, till I am convinced to the contrary, I hold to be great guns.

     Spice is becoming frist-rate: she is the principal patient under the ophthalmoscope, and turns her eyes at   command, so as to show the tapetum, the optic nerve, or any required part. Dr. Bowman, the great oculist, came    to see the sight, and when we were out of town he came again and brought Donders of Utrecht with him to visit   Spice.

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TO C. H. CAY, Esq.

                                                                        Glenlair, 14th October 1865.

     . . . I hope you keep your conscience in good order, and do not bestow more labour on erroneous papers than is  useful to the youth who wrote it. Always set him to look for the mistake, if he prefers that to starting fresh, for to  find your  own mistake may sometimes be profitable, but to seek for another man's mistake is weariness to the flesh.

     There are three ways of learning props.—the heart, the head, and the fingers; of these the fingers is the thing for    examinations, but it requires constant practice. Nevertheless the fingers have a fully better retention of methods    than the heart has. The head method requires about a mustard seed of thought, which, of course, is expensive, but    then it takes away all anxiety. The heart method is full of anxiety, but dispenses with the thought, and the finger    method requires great labour and constant practice, but dispenses with thought and anxiety together.

     We have had very fine weather since you went away, but I was laid up for more than three weeks with erysipelas    all over my head, and got very shaky on my pins. But I have been out for a fortnight, and riding regularly as of old,   which is good for Katherine after the nursing, and I eat about double what any man in Galloway does, and know  nothing of it in half an hour; but my legs are absorbing the beef as fast as it is administered.