March 11, 1887.] 



SCIJENCE, 



231 



LETTERS TO THE EDITOR. 



*t* Correspondents are requested to be as brief as possible. The 

 writer's name is in all cases required as proof of good faith. 



The failure of foreign trees on American soil. 



Allow me to enter a respectful protest against the 

 sweeping judgment of Professor Sargent in condem- 

 nation of foreign trees, which you publish approv- 

 ingly in your issue of March 4. Though there is, no 

 doubt, a great difference between the climate of this 

 continent and that of Europe, and though unques- 

 tionably tree-growth is most dependent upon climat- 

 ic conditions, yet it would be unwarrantable, from 

 its failure in one place or even several places in this 

 country in ornamental plantations, to generalize 

 upon the adaptability of an exotic species for forestry 

 use. It seems to be generally overlooked, if not un- 

 known, in this coiiutry, that forestry and arboricul- 

 ture, or tree-planting as practised by the horticultur- 

 ist or landscape-gardener, are not the same thing, 

 but in their objects, and consequently in their 

 methods and results, are entirely different. While 

 in ornamental planting the individual tree is the ob- 

 ject, and its form in its unity and the development 

 of its beauty is the aim of the planter, forestry has 

 to do with an aggregate of trees, which, properly 

 placed and grouped together, grow and develop very 

 differently from the single tree, or even group of 

 trees, on the lawn. The European larch, even in its 

 native country, does not make a desirable lawn-tree 

 in every locality, and, coming originally from the 

 highest mountain elevations, even as a forest-tree, it 

 requires, when grown upon the plain, particular con- 

 ditions and special management to secure a thrifty 

 growth, and the quality and quantity of timber for 

 which the tree is noted. I have often pitied those in 

 this country who have expected these results with- 

 out paying attention to the requirements of the tree. 

 As to the Norway spruce, of which Professor Sargent 

 speaks so disparagingly, I have not seen a finer or- 

 namental conifer of its kind on this side of the Atlan- 

 tic : and though, as is the case with all the conifers, 

 a time arrives when it loses its peculiar beauty, I 

 doubt whether it does so sooner than any others, 

 while, as a forest-tree, it needs only proper condi- 

 tions and management, I venture to say, in order to 

 attain the size and quality which it shows in its 

 native country. Plant the Norway spruce in dense 

 groves, on a northern or north-western exposure, 

 with the Eiiropean larch sparingly interspersed, and 

 no planter will live long enough to see these two, thus 

 united, fail in their onward development. 



The Scotch-pine, on poor but deep sands on the 

 western prairies, I am sure will make useful timber 

 sooner than the white-pine. The white-pine was in- 

 troduced into Germany on large areas about ninety 

 years ago. G-rowing with great rajjidity, and yield- 

 ing astonishing quantities of wood per acre, the 

 quality of the wood was found to be very inferior 

 until recent years. Experiments have lately shown 

 that the white-pine requires ninety years to make 

 wood of as good quality as the Scotch-pine will pro- 

 duce in seventy years under similar conditions, just 

 as different grains will require different lengths of 

 season in which to mature. These experiments and 

 the many similar ones which could be cited should 

 teach us to be chary of generalizations upon our 

 scanty experiences in forestry in this country. 



Of the European willows, so far as osier-growing 

 is concerned, only one, Salix purpurea, seems to 



have been found ' adapted to our climate, while 

 several native ones promise success if properly 

 treated. 



While I am a most earnest advocate of seeking for 

 the best in that which we have ourselves, and while I 

 advise the planting first of our native trees, with a 

 special study of their requirements, I must deprecate 

 any know-nothing movement against the good things 

 which we may import. Especially let us remember 

 that New England constitutes, territorially and cli- 

 matically, but a very small part of our country, and 

 that conclusions drawn from experiments there may 

 not be applicable to other portions of it. 



B. E. Feenow.. 

 Washington, March 7. 



Inertia-force. 



I had thought that my pamphlet, ' Elementary 

 ideas,' etc., might awaken discussion, and possibly 

 bring about a better understanding among teachers- 

 of physics as to the interpretation of certain familiar 

 terms. The discussion has evidently begun. Let 

 us not despair of the better understanding. 



Having made, however, one direct attempt to ex- 

 plain to Professor MacGregor my use of the term 

 ' inertia-force,' with the sorry result of disgusting^ 

 him by the use of " language which is not the cur- 

 rent language of dynamics," I shall for the moment 

 adopt a different course, and find a little fanlt witk 

 his way of stating things. 



Professor MacGregor accepts fully the doctrine- 

 stated by Maxwell in a passage quoted in my first 

 letter, that " all force is of the nature of stress, that 

 stress exists only between two portions of matter," 

 and that " the stress is measured numerically by the 

 force exerted on either of the two portions of mat- 

 ter." I will undertake to show wherein his reason- 

 ing seems to me to be inconsistent with this doctrine. 

 He takes my illustration of a railway-train which ia 

 being set in motion by a locomotive, and says, " If 

 F is the pull of the locomotive, R the frictional re- 

 sistance, Mthe mass of the train, and a its accelera- 

 tion, we have undoubtedly, by Newton's second law 

 of motion, 



a = (F- R) ~ M." 



To this every one will agree. Now, with Professor 

 MacGregor's permission, I will put this equation in 

 the form 



F= R + aM. 



F is, by his own statement, a force, — the force ex- 

 erted by the locomotive on the train. By the doc- 

 trine stated by Maxwell, which Professor MacGregor 

 accepts, the force exerted hy the train on the locomo- 

 tive is also equal to F. It is therefore equal to, and 

 may be expressed by, the terms R + aM. Now, one 

 part of this force, the part R, is accounted for by the 

 resistance of friction transmitted through the train 

 to the coupling of the locomotive. How shall we ac- 

 count for the other part of the whole force exerted 

 by the train on the locomotive, the part aM ? I call 

 it the inertia-force, — the force, or resistance, which 

 the train, by virtue of its inertia, exerts on the loco- 

 motive which is setting it in motion. I think I can 

 be XDcrsuaded to drop the term ' inertia-force,' if a 

 more accurately descriptive one can be adopted ; but 

 Professor MacGregor, if I understand him, does not 

 object to the term merely. He denies that the train 

 offers any resistance by virtue of its inertia. But in. 



