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nce the time of Anaxagoras； that philosopher's idea of the molecular nature of matter had gained fairly wide currency。 As to the expansive power of gas； which Hero describes at some length without giving us a clew to his authorities； we may assume that Ctesibius was an original worker； yet the general facts involved were doubtless much older than his day。 Hero； for example； tells us of the cupping…glass used by physicians； which he says is made into a vacuum by burning up the air in it； but this apparatus had probably been long in use； and Hero mentions it not in order to describe the ordinary cupping…glass which is referred to； but a modification of it。 He refers to the old form as if it were something familiar to all。 Again； we know that Empedocles studied the pressure of the air in the fifth century B。C。； and discovered that it would support a column of water in a closed tube； so this phase of the subject is not new。 But there is no hint anywhere before this work of Hero of a clear understanding that the expansive properties of the air when compressed； or when heated； may be made available as a motor power。 Hero； however； has the clearest notions on the subject and puts them to the practical test of experiment。 Thus he constructs numerous mechanisms in which the expansive power of air under pressure is made to do work； and others in which the same end is accomplished through the expansive power of heated air。 For example； the doors of a temple are made to swing open automatically when a fire is lighted on a distant altar； closing again when the fire dies outeffects which must have filled the minds of the pious observers with bewilderment and wonder； serving a most useful purpose for the priests； who alone； we may assume； were in the secret。 There were two methods by which this apparatus was worked。 In one the heated air pressed on the water in a close retort connected with the altar； forcing water out of the retort into a bucket； which by its weight applied a force through pulleys and ropes that turned the standards on which the temple doors revolved。 When the fire died down the air contracted； the water was siphoned back from the bucket； which； being thus lightened； let the doors close again through the action of an ordinary weight。 The other method was a slight modification； in which the retort of water was dispensed with and a leather sack like a large football substitued。 The ropes and pulleys were connected with this sack； which exerted a pull when the hot air expanded； and which collapsed and thus relaxed its strain when the air cooled。 A glance at the illustrations taken from Hero's book will make the details clear。 Other mechanisms utilized a somewhat different combination of weights； pulleys； and siphons； operated by the expansive power of air； unheated but under pressure； such pressure being applied with a force… pump； or by the weight of water running into a closed receptacle。 One such mechanism gives us a constant jet of water or perpetual fountain。 Another curious application of the principle furnishes us with an elaborate toy； consisting of a group of birds which alternately whistle or are silent； while an owl seated on a neighboring perch turns towards the birds when their song begins and away from them when it ends。 The 〃singing〃 of the birds； it must be explained； is produced by the expulsion of air through tiny tubes passing up through their throats from a tank below。 The owl is made to turn by a mechanism similar to that which manipulates the temple doors。 The pressure is supplied merely by a stream of running water； and the periodical silence of the birds is due to the fact that this pressure is relieved through the automatic siphoning off of the water when it reaches a certain height。 The action of the siphon； it may be added； is correctly explained by Hero as due to the greater weight of the water in the longer arm of the bent tube。 As before mentioned； the siphon is repeatedly used in these mechanisms of Hero。 The diagram will make clear the exact application of it in the present most ingenious mechanism。 We may add that the principle of the whistle was a favorite one of Hero。 By the aid of a similar mechanism he brought about the blowing of trumpets when the temple doors were opened； a phenomenon which must greatly have enhanced the mystification。 It is possible that this principle was utilized also in connection with statues to produce seemingly supernatural effects。 This may be the explanation of the tradition of the speaking statue in the temple of Ammon at Thebes。 ｛illustration caption = DEVICE FOR CAUSING THE DOORS OF THE TEMPLE TO OPEN WHEN THE FIRE ON THE ALTAR IS LIGHTED （Air heated in the altar F drives water from the closed receptacle H through the tube KL into the bucket M； which descends through gravity； thus opening the doors。 When the altar cools； the air contracts； the water is sucked from the bucket； and the weight and pulley close the doors。）｝ ｛illustration caption = THE STEAM…ENGINE OF HERO （The steam generated in the receptacle AB passes through the tube EF into the globe； and escapes through the bent tubes H and K； causing the globe to rotate on the axis LG。）｝
The utilization of the properties of compressed air was not confined； however； exclusively to mere toys； or to produce miraculous effects。 The same principle was applied to a practical fire…engine； worked by levers and force…pumps； an apparatus； in short； altogether similar to that still in use in rural districts。 A slightly different application of the motive power of expanding air is furnished in a very curious toy called 〃the dancing figures。〃 In this； air heated in a retort like a miniature altar is allowed to escape through the sides of two pairs of revolving arms precisely like those of the ordinary revolving fountain with which we are accustomed to water our lawns； the revolving arms being attached to a plane on which several pairs of statuettes representing dancers are placed； An even more interesting application of this principle of setting a wheel in motion is furnished in a mechanism which must be considered the earliest of steam…engines。 Here； as the name implies； the gas supplying the motive power is actually steam。 The apparatus made to revolve is a globe connected with the steam…retort by a tube which serves as one of its axes； the steam escaping from the globe through two bent tubes placed at either end of an equatorial diameter。 It does not appear that Hero had any thought of making practical use of this steam… engine。 It was merely a curious toynothing more。 Yet had not the age that succeeded that of Hero been one in which inventive genius was dormant； some one must soon have hit upon the idea that this steam… engine might be improved and made to serve a useful purpose。 As the case stands； however； there was no advance made upon the steam motor of Hero for almost two thousand years。 And； indeed； when the practical application of steam was made； towards the close of the eighteenth century； it was made probably quite without reference to the experiment of Hero； though knowledge of his toy may perhaps have given a clew to Watt or his predecessors。
｛illustration caption = THE SLOT…MACHINE OF HERO （The coin introduced at A falls on the lever R； and by its weight opens the valve S； permitting the liquid to escape through the invisible tube LM。 As the lever tips； the coin slides off and the valve closes。 The liquid in tank must of course be kept above F。）｝ In recent times there has been a tendency to give to this steam…engine of Hero something more than full meed of appreciation。 To be sure； it marked a most important principle in the conception that steam might be used as a motive power； but； except in the demonstration of this principle； the mechanism of Hero was much too primitive to be of any importance。 But there is one mechanism described by Hero which was a most explicit anticipation of a device； which presumably soon went out of use； and which was not reinvented until towards the close of the nineteenth century。 This was a device which has become familiar in recent times as the penny…in…the…slot machine。 When towards the close of the nineteenth century some inventive craftsman hit upon the idea of an automatic machine to supply candy； a box of cigarettes； or a whiff of perfumery； he may or may not have borrowed his idea from the slot…machine of Hero； but in any event； instead of being an innovator he was really two thousand years behind the times； for the slot…machine of Hero is the precise prototype of these modern ones。 The particular function which the mechanism of Hero was destined to fulfil was the distribution of a jet of water； presumably used for sacramental purposes； which was given out automatically when a five… drachma coin was dropped into the slot at the top of the machine。 The internal mechanism of the machine was simple enough； consisting merely of a lever operating a valve which was opened by the weight of the coin dropping on the little shelf at the end of the lever； and which closed again when the coin slid off the shelf。 The illustration will show how simple this mechanism was。 Yet to the worshippers； who probably had entered the temple through doors miraculously opened； and who now witnessed