46 SIR ROBERT CHRISTISON AND DR CHRISTISON ON THE 



ments might not, from some difference of manipulation, disagree with his. 

 Accordingly, with the aid of my brother, I remeasured early in 1882 the forty- 

 one trees in the Botanic Garden measured by Sir Robert at the end of the 

 growing season in 1881. The result was satisfactory. In nineteen instances 

 there was no appreciable difference between the two measurements ; in seven- 

 teen the difference did not exceed a twentieth of an inch ; in three it 

 amounted to a tenth, and in two to a seventh of an inch. Thus in only five 

 cases were the discrepancies so great as to be of material consequence ; and, 

 on investigation, these discrepancies were found to be evidently due either to 

 extreme roughness or a tendency to scale in the bark. So great a degree of 

 accuracy as this however cannot be obtained with ordinary tapes. I have found 

 some of the inches marked on these a tenth of an inch too large, others a tenth 

 too small. Another source of error with them is the terminal ring with the 

 fastenings by which it is attached to the tape. If the measurement be taken over 

 the ring, and it happens to be sunk in a depression of the tree, no error results ; 

 but if the ring be on a projection of the bark, its bulk may cause an error in 

 excess amounting to a twentieth or even a tenth of an inch. A different result 

 from either of these will probably be got if the measurement is kept clear of 

 the ring altogether. In the early part of his experiments Sir Robert used a 

 tape, painted so as to avoid stretching, and graduated by himself; an extra 

 inch graduated to tenths served for taking the fractions of an inch, so that it 

 was unnecessary to graduate the tape throughout into tenths. But mistakes 

 were apt to arise from the necessity of reckoning the tenths in a direction con- 

 trary to the numbering of the inches, and ultimately he used a steel tape, 

 graduated throughout to tenths, made specially for him by Messrs Chesterman. 

 This is certainly the kind most to be recommended. 



I. Annual Observations. • 



Following Sir Robert's example, I give the increments for 1881 and 1882 

 in a tabular form, along with those already published for previous years. As in 

 the course of time however several of the trees originally selected have ceased 

 to be eligible, I have found it necessary to remodel the table to a considerable 

 extent. Thus the Scots fir, No. 19 in his list, and the Picea Lowei, 32, having 

 ceased to grow, have been cut down; the Scots firs, 11, 36, 37, have also 

 ceased to grow for three years ; and the yew, 47, is almost in the same predica- 

 ment. As it was obviously useless to retain these, they have been struck out ; 

 and the Pinus Laricio, 17, the aged sycamore, 13, and walnut, 14, having bark 

 either so scaly or so rugged as to be unsuitable for minute measurements, have 

 shared the same fate. In compensation for these losses in the Botanic Garden, 

 a larger number of trees growing at Craigiehall, five miles from Edinburgh, have 

 been selected for observation and added to the list. No confusion need be 

 feared from these changes in making comparisons with former years, as the 



