From Horace Darwin to G. H. Darwin [29 September? 1877]

Dear George.

I send you a drawing of a dodge of keeping a chamber at a uniform temperature by a gas flame. The principle is pitch, & a thing just like your experimental machine at Cam.. The drawings I send are not quite like I should make it but will do to explain; it may amuse you to see it as it is cribbed from you. Tell Frank we will make one for him if he wants it, with night light or a lamp. I of course think it rather good. The clock arrangement Fig 2 will be very much altered but more difficult to explain. I’m very glad you have got me rooms. Where is Peas Hill; SW corner of market place.

Yours ever | H Darwin

Please keep drawings

Fig I

a is a pully round which a string passes going to another pully which is driven slowly by a clock at a uniform rate

b a wire on wh a is fixed

c, d, e a brass tube with a flange e running round it   this tube fits into a hole in the top of the box to be kept at a uniform temperature & the flange stops it falling through the hole. The shading up as high as d represents water to. conduct the heat to the inner part of apparatus.

The top part c consists of two arms bending over to support the bearing f for the wire b

g h k is another brass tube made so as to screw together at h & is filled with pitch up to l; the wire b passes through two bearing m n in this tube to steady it.

A string P is wound round the top of this brass tube, and a constant force keeps this string in tension so as to give the tube g h k a constant moment about its axis. This tube has a point at r which can turn in a conical hole in the bottom of outer tube . c e d.

The clock tends to wind up the string P by turning the tube g through the stiffness of the pitch. but the tube g will tend to turn the other way by the tension of the string. Hence as one temperature the tube g will stay still, & turn in one direction if it gets colder & in the other direction if hotter, wh may be made to turn gass or a lamp off and on.

See Fig 2

To the right is the hot chamber with Fig I let in at the top   A simple form of clock gives the uniform motion to the wire in Fig I. And a string going over some pulleys & fastened to a scale pan gives the constant force. Just above the scale pan is fastened the end of a lever A B which turns about A (See (fig 3). So as the scale pan goes up or down, i.e. at chamber gets hotter or cooler, this lever turns about A; this acts like a nut crackers on an indian rubber pipe & thus alters the supply of gas to heat the chamber. it must be arranged so at never to quite put the gass out only make it very low   The right weight to put into scale pan must be found by trial.

Fig 1

Vertical Section

[DIAG HERE]

Fig 2.

[DIAG HERE] ["A square piece of wood to which clock is fixed | Pulley fixed to square board | Strings | Clock | Clock weight | Scale pan, pulling weights in here lowers temperature in hot chamber | Vertical elevation & part section | Lever turning abt A | Hot chambers | Fig I"]

Fig 3.

[DIAG HERE] [annotated: "A", "B", and "Section of indian rubber pipe."]