CHAP. X.] MARRIAGE ENGAGEMENT. 303 



been hard at mathematics. In fact I set myself a great 

 arithmetical job of calculating the tangential action of two 

 rings of satellites, and I am near through with it now. I 

 have got a very neat model of my theoretical ring, a credit 

 to Aberdeen workmen. Here is a diagram, but the thing is 

 complex and difficult to draw : 



Two wheels turning on parallel parts of a cranked axle ; 

 thirty-six little cranks of same length between corresponding 

 points of the circumferences ; each carries a little ivory 

 satellite. 1 



To Miss CAY. 



129 Union Street, 



18th February 1858. 



DEAR AUNT This comes to tell you that I am going 

 to have a wife. 



I am not going to write out a catalogue of qualities, as 

 I am not fit ; but I can tell you that we are quite necessary 

 to one another, and understand each other better than most 

 couples I have seen. 



Don't be afraid ; she is not mathematical ; but there 

 are other things besides that, and she certainly won't stop 

 the mathematics. The only one that can speak as an eye- 

 witness is Johnnie, and he only saw her when we were both 

 trying to act the indifferent. We have been trying it since, 

 but it would not do, and it was not good for either. 



So now you know who it is, even Katherine Mary Dewar 

 (hitherto). I have heard Uncle Eobert speak (second-hand) 

 of her father, the Principal. Her mother is a first-rate lady, 

 very quiet and discreet, but has stuff in her to go through 

 anything in the way of endurance. ... So there is the 

 state of the case. I settled the matter with her, and the 

 rest of them are all conformable. 



I hope some day to make you better acquainted. I can 

 hardly admit that Johnnie saw her at all, not as he will 

 when she appears in a true light. . . . For the present you 



1 The sketch, which follows corresponds to the model which is 

 preserved in the Cavendish Laboratory. See Part II. ^ 



