Note 72 — Chapter III

A careful discussion which I have recently undertaken of the entire series of reciprocal observations made during the three seasons’ work shows that k varies according to the altitude of the lines and according as the lines pass over land or sea. I hope later to publish a separate account of this discussion, which has an important bearing on the vertical distribution of temperature in the air. But I may here state that for overland lines the formula k = 0·1503 + 0·0000164 (h − 830), where h is the mean altitude of the line in metres, gives a good agreement with the observations; while on oversea lines at altitudes above 100 metres k is always very near to 0·135. The errors in the resulting altitudes of intersected points due to the uniform employment of k = 0·13, are very trifling, and will seldom exceed one or two metres; for example, at a distance of thirty kilometres the substitution of k = 0·16 for k = 0·13 will only change the altitude by two metres.