346 VIII. ALTITUDE OF SPECIES. 



examined, and some of the old ground has been re-examined ; so that 

 the present list is now drawn from notes considerably more extended. 

 Increased experience, it may be trusted, has led also to increased 

 accuracy in processes so liable to uncertainty and error on the part of 

 those who seek to determine altitudes. Unless otherwise shown, as by 

 the inverted commas or added authority, the present writer must be held 

 responsible for all the heights stated in the list. His datum during 

 measurements has seldom been the sea-level. The summit of Ben 

 Lawers, at 3950 feet, has given one datum ; with which to compare the 

 altitudes in Mid Perth, &c. Kingussie, at 750 feet, has given a second 

 datum; with which to compare a wide mountainous tract about Loch 

 Erricht. And the village of Castletown in Braemar, at 1070 feet, has 

 been a third datum ; used for the south-west of Aberdeenshire and adja- 

 cent portions of Perthshire and Forfarshire. Those three heights have 

 been published on authority held safe and reliable. 



But in assuming the alleged altitudes of those three places to be 

 quite correct as starting points, it is not pretended that the hundreds of 

 other altitudes have all been ascertained with like accuracy. On the 

 contrary, the reader is here requested to look back to an explanation 

 about the diflference between measured and estimated altitudes, inci- 

 dentally given on pages 213, 214 of volume third, under the head of 

 Cynosurus cristatus. And if desirous of further information as to the 

 manner of taking the heights of stations, he will find it more fully 

 explained in the volume of the London Journal of Botany, above 

 referred to. It may be shortly stated here, that many of the figures in- 

 troduced into the list are estimates formed in a similar manner with the 

 one explained for Cynosurus cristatus. The heights of two spots being 

 taken as precisely as the quality of the instruments and state of weather 

 would allow, the altitudes of plants noted between those two spots were 

 estimated or guessed according to their successive appearance between 

 the two spots, and their apparent distances from one or both of them. 

 The altitudes for various plants noted at or about the particular spots so 

 measured, — often a hill-summit or table-land, — or the true heights of 

 which were supposed to be known otherwise on some good authority, 

 must be deemed more closely correct. But in truth all modes of taking 

 heights indirectly, through the aid of instruments which indicate simply 

 the pressure of the atmosphere and its temperature, as data from which 

 the heights are to be calculated, are liable to grave errors occasionally ; 

 although with due care they may sufiice to give fair approximations, 



