Second in number, however, are the impressions received through the sense of hearing, and consequently the memory stores away a great number of sound impressions. In some cases the impressions of sight and sound are joined together, as for instance in the case of words, in which not only the sound but the shape of the letters composing the word, or rather the word-shape itself, are stored away together, and consequently are far more readily remembered or recollected than things of which but one sense impression is recorded. Teachers of memory use this fact as a means of helping their students to memorize words by speaking them aloud, and then writing them down. Many persons memorize names in this way, the impression of the written word being added to the impression of the sound, thus doubling the record. The more impressions that you can make regarding a thing, the greater are the chances of your easily recollecting it. Likewise it is very important to attach an impression of a weaker sense, to that of a stronger one, in order that the former may be memorized. For instance, if you have a good eye memory, and a poor ear memory, it is well to attach your sound impressions to the sight impressions. And if you have a poor eye memory, and a good ear memory it is important to attach your sight impressions to your sound impressions. In this way you take advantage of the law of association, of which we have told you. Under the sub-class of sight impressions, are found the smaller divisions of memory known as memory of locality; memory of figures; memory of form; memory of color; and memory of written or printed words. Under the sub-class of sound impressions are found the smaller divisions of memory known as memory of spoken words; memory of names; memory of stories; memory of music, etc. We shall pay special attention to these forms of memory, in succeeding chapters. The second general class of memory,--memory of ideas,--includes the memory of facts, events, thoughts, lines of reasoning, etc., and is regarded as higher in the scale than the memory of sense impressions, although not more necessary nor useful to the average person. This form of memory of course accompanies the higher lines of intellectual effort and activities, and constitutes a large part of what is known as true education, that is education which teaches one to think instead of to merely memorize certain things taught in books or lectures.