From Horace Darwin to G. H. Darwin 9 March [1881?]

Dear George.

I have been here most of this week but Ida has stayed at Bry ⃞ to help to look after Claudie. He is getting on well, & is not so bad a Cassidy made out at first, but it is quite possible I fear that his limp may be effected. I know very little Cam. news I dont even know how Challis is, about the same I expect but I don’t know. Miss Gladstone has accepted the head ship of the South Hall, & the Sidgwicks are going abroad for a year after June when Miss G. takes possession. I saw F.M.B for an hour yesterday & he was cheerful & talkative, but very weak yet, he had walked accross the room for the first time a day or two before. He is going to Salisbury Villars for a few days & then will go to London for a bit & after that his plans are uncertain. When I was in town I saw Douglas & he does really seem to want an instrument for showing how much a lighthouse shakes; I have ideas about wh. I want to tell you, but I believe I told you this before! I have invented a hot air engine wh. wont act, that’s to say I suppose it wont act but I dont understand it: if it did act, it would only be about $\frac{1}{10000}$ of a horse power, but I like it all the same.

A B is a pipe, [DIAG HERE] suppose you blow air in at A; disregarding friction & suppose there is a steady current of air through the pipe there will be less pressure at C than at A or B as the pipe is smaller at C. Therefore at D the air is expanding, and at E it is being compressed.

Now you cool the pipe at E & heat it at D. Follow a particle of of air as it passes through the pipe.

1^st. it is heated whilst expanding (isothermal) D

2^nd. " " expanded without heating (adiabatic) D′

3^rd. " " compressed whilst being cooled (isothermal) E

4 compressed without being cooled (adiabatic) E′

Is not this a heat engine?

Join the two ends of the tube & make the air do work by driving an engine of some sort.

Your affec brother | Horace Darwin