My dear aunt Jessie,

It seems very odd to me that I should have been all this time without writing to you, but I have been so helpless and unable to do anything that I never had the energy to write, though I was often thinking of it. Now I am quite well and strong and able to enjoy the use of my legs and my baby, and a very nice looking one it is, I assure you. He has very dark blue eyes and a pretty, small mouth, his nose I will not boast of, but it is very harmless as long as he is a baby. Elizabeth went away a week too soon while he was a poor little wretch before he began to improve. She was very fond of him then, and I expect she will admire him as much as I do in the summer at Maer. He is a sort of grandchild of hers

Charles and I were both very much pleased at having a visit from Papa, and he looked comfortable in his armchair by the fire, and told us that Gower St. was the quietest place he had ever been at in his life; and Elizabeth finds it very quiet after Maer, though she had a little private dissipation of her own, dining and going to parties at the Inglis's, Dr Holland's, Georgina [Aldersonjs, etc. but she has a different sort of bustle at Maer.

THE EQUATIONS OF MASS MOTION

209. In the last chapter some insight was obtained into the way in which pressure is exercised by a collection of moving molecules. We shall now attempt to carry further the study of the physical properties of a medium consisting of moving molecules, especially with reference to the similarities between the behaviour of such a medium and a continuous fluid.

We shall no longer consider only problems in which the gas is in a steady state, so that the law of distribution is Maxwell's law at every point. We shall consider problems in which the law of distribution of velocities is the general law f(u, v, w), and it will no longer be assumed that the law of distribution is the same at every point: we shall suppose the law of distribution to depend on x, y, z, as well as on u, v, w. When it is required to indicate this we shall write the law of distribution in the form f (u, v, w, x, y, z).

The number of molecules having their centres within the element of volume dxdydz surrounding the point x, y, z, and having their velocities within a range dudvdw surrounding the values u, v, w, will be vf (u, v, w, x y, z) dudvdwdxdydz(403).

In this expression vwill in general be a function of x, y, z, and will be evaluated at the same point as f. It is often convenient to think of the product vf as a single variable.

In 1847 the three sisters Jessie Sismondi, and Emma and Fanny Allen, were moving house. There appears to have been much heart-searching as to whether they should uproot altogether and follow their favourite nieces Elizabeth, Charlotte, Emma, and Fanny Hensleigh to the home counties, but eventually they decided that it was best to live and die where they had been born and where Fanny and Emma had spent most of their lives—a wise decision I imagine.

Jessie Sismondi writes to Elizabeth Wedgwood (June 7, 1847):

This year, too, Sarah Wedgwood, having left Camp Hill, her Staffordshire home, came to live at Down to be near my mother. She took ‘Petleys’ from Sir John Lubbock, and lived there till her death. Her three servants were the dear friends of us all as children, and whenever life was a little flat at home, a visit to the kitchen at aunt Sarah's to see Mrs Morrey, her sister Martha, and the manservant Henry Hemmings, was always delightful.

61. The problem of the present chapter will be to consider the relation between the methods of procedure adopted in Chapters II and III.

The discussion of Chapter II was based upon certain questions of probability, and an answer to these questions was made possible and was obtained by the help of the assumption of molecular chaos enunciated in § 15.

The discussion of Chapter III also rested, although in a different sense, upon the theory of probability. The generalised space filled with fluid supplied a basis for the calculation of probabilities, and as the motion of the fluid was proved to be steady-motion, it followed that this basis was independent of the time. For the present, we continue to take this generalised space as the basis of probability calculations. The question “What is the probability that a system satisfies condition p?” will be taken to mean: “For what proportion of the generalised space is condition p satisfied?” The further question: “Given that a system satisfies condition p, what is the probability that it also satisfies condition q?” will be interpreted to mean: “A point is selected at random from all those parts of the generalised space in which condition p is satisfied: what is the probability that at this point condition q also is satisfied?”