Sir, - With reference to Frank Kilfeather's article ("Moustaches a loss leader for imbibers of the black stuff" February 25th), your readers may be interested in two further subtleties of the Guinness pint that we discovered when I was head brewer and subsequently technical development director at the St James's Gate brewery of Arthur Guinness Son & Co. (Dublin) Ltd.
Due to the effect of atmospheric pressure, the weather and altitude above sea level affect the size of the head. When the pressure is low the head size increases, and correspondingly when the pressure is high the head size decreases. The effect in Ireland is quite small in consumer terms, amounting to a change of one millimetre for every 10 millibars/hectopascals change in atmospheric pressure - equivalent to a maximum increase or decrease of about a sixth of an inch for an extreme high or low. Nevertheless, the quality standards to which the Guinness brewery works are so stringent that it was found necessary to introduce a barometric correction factor into head size measurements.
However, in the case of high altitude draughting of Guinness, such as in Johannesburg and Denver, the lower pressure produces an increase of about 1618mm - about three quarters of an inch - which requires extra cooling or a different shape of glass to avoid the head height becoming too large.
Moving from the present to the future, the Guinness consumer in orbiting space hotels, where artificial gravity from rotation would be much less than on Earth, would require considerable patience because the pint would take very much longer to settle - perhaps there is scope here for a new Guinness ad on all the things you might do in orbit whilst waiting for your pint to settle! - Yours, etc.,
Tony J. Carey, Enniskerry, Co Wicklow.