A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Bill Morris
Date: 2015 Apr 19, 21:07 -0700
I agree with Geoffrey that if the clock was sealed hermetically in a case it would have been isolated from changes in atmospheric pressure, and I wonder how then the clock was wound. Variation in atmospheric pressure affects the weight of the pendulum bob, giving an error of about 0.012 sec/day/mmHg. It is as if the gravitation force had changed. Frictional changes in themselves appear to be negligible but stirring the air in a case removes energy from the pendulum and also affects its rate. If it were also housed in a room of relatively constant temperature, something that is easy for us with thermostatically controlled room temperature, then temperature compensation would be less demanding. Although Harrison's grasshopper escapement had lignum vitae pallets (like the reproduction), the self-lubricating property of the wood is not needed, because Harrison aimed for there to be no sliding or rolling contact at the pallet faces. They are either engaged with a tooth or not. The escapement is not dead beat, as there is considerable recoil, nor is it detached from the driving force. Laurie Penman writes in "Practical Clock Escapements" 1998,"However, the fact that the pendulum and the escapement are never free of one another , and that there is no sliding friction between parts of the escapement, produces a device where the components of the escapement and circular error can be controlled and balanced so that the effects of changing frictional load and barometric changes on the rate of the clock are negated." I am not at all sure that I follow what he means.
In a clock in which the oscillator is a balance wheel and spring, the period of oscillation is independent of its amplitude (unlike a pendulum), provided that there is no interference from the driving force which keeps it going. In the detached chronometer escapement, the balance wheel is free from connection with the rest of the mechanism for a large part of its oscillation, so that variations in the driving force, and hence the amplitude of oscillation, should not sensibly affect its period. However, a heavy temperature compensated chronometer balance wheel in which the rim is split, tends to lose when the amplitude of oscillation increases, because centrifugal forces increase the diameter and hence the moment of intertia. Hamilton's "ovalising balance" was not split and so was free of this problem. Of course, frictional forces at the pivots of the balance are another interfering factor, but a relatively minor one. There seems to be no very obvious correlation of rate and barometric pressure in marine chronometers. If there is an effect it is probably very small.
I will be interested to see whether Geoffrey's chronometer can match the Burgess clock and will also see how my Hamilton Model 21 performs.
As an aside, I am always slightly irritated when I read of Harrison as being the "father of the chronometer". Maybe he was the father of _a_ chronometer and showed it could be done, and we may admire the complexity and fascination of his machines. but his was a developmental dead end which could almost be described as a triumph of workmanship over design. The only original feature of his chronometers to be found in modern mechanical chronometers is his maintaining power, that which keeps the clock going as it is being wound.