The elms, taken as a class, are much alike. There is more resemblance between the species than between species of oaks or pines, yet some difference exists between elms. This holds not only between different species, but between individuals of the same species. Climate, situation, and soil have much to do with the character of the wood of the same species. So great is the difference at times that fairly good judges of timber are deceived as to the species. A tree growing on dry, rocky soil produces wood quite different from one on rich, deep, well-watered soil. Not only is the wood of one different from that of the other, but the appearances of the standing trees are not alike. The differences may not show in leaves, flowers, and fruit as much as in the shapes and sizes of trees, and the habit of the branches. White elm is by common consent the type of the genus, the standard by which the other species are measured. It is proper to compare certain properties and characters of other elms with white elm, in order that a general view of all may be had. The dry weights, per cubic foot, of wood are as follows: White elm 40.54 pounds, slippery elm 43.35, cedar elm 45.15, cork elm 45.26, and wing elm 46.69. Figures which show the weight of the southern red elm (_Ulmus serotina_) are not available. White elm is thus shown to be lightest of the group. Its breaking strength averages 12,158 pounds per cubic inch, under the usual tests by which the strength of woods is determined. Reduced to everyday language that means that 12,158 pounds would just break a white elm stick, 2⅝ inches square, and resting on supports twelve inches apart. That is the meaning of “breaking strength,” or “modulus of rupture,” as the term is used in engineering text books relating to woods. The following figures for the breaking strength of other elms make comparisons with white elm easy: Cedar elm 11,000; wing elm 11,162; slippery elm 12,342; cork elm (often called rock elm) 15,172. It is shown that two elms are stronger and two weaker than white elm. This wood rates very little below white oak in strength. The different species of elms vary considerably in stiffness, or the ability to spring back when bent. This factor is expressed by engineers in high figures, is purely technical, and is based on a wood’s ability to stretch and regain its former position. The only service which the figures can render to the layman is to furnish a basis for comparing one wood with another. The stiffer a wood, the greater its resistance to an effort to stretch it lengthwise. White elm’s measure of stiffness (modulus of elasticity) is 1,070,000 pounds per square inch; wing elm 853,000; cedar elm 981,000; slippery elm 1,318,000; cork elm 1,512,000. It is shown here, as was shown in the figures representing the strength of the elms, that two species rate above and two below white elm in stiffness.