July 23, 1891] 



NATURE 



277 



perature with a pull equivalent to a negative pressure of 

 25 atmospheres, by completely filling a bulb with 

 alcohol, and then cooling it. The alcohol in con- 

 tracting strains the bulb inwards ; and finally, when the 

 tension becomes very great, parts from the glass with a 

 sharp " click." 



To realize a portion of the other bend of the curve, an 

 experiment has been devised by Mr. John Aitken. It is 

 as follows : — If air — that is, space, for the air plays a 

 secondary part — saturated with moisture be cooled, the 

 moisture will not deposit unless there are dust-particles 

 on which condensation can take place. It is not at first 

 evident how this corresponds to the compressing of a gas 

 without condensation. But a glance at the figure will 

 render the matter plain. Consider the isothermal 175° for 

 ether, at the point marked A. If it were possible to lower 

 the temperature to 160% without condensation, keeping 

 volume constant, pressure would fall, and the gas would 

 then be in the state represented on the isothermal line 

 160°, at G : that is, it would be in the same condition as 

 if it had been compressed without condensation. 



You saw that a gas, or a liquid, is heated by com- 

 pression ; a piece of tinder was set on fire by the heat 

 evolved on compressing air. You saw that condensation 

 of ether was brought about by diminution of pressure — 

 that is, it was cooled. Now, if air be suddenly expanded, 

 it will do work against atmospheric pressure, and will 

 cool itself. This globe contains air ; but the air has 

 been filtered carefully through cotton-wool, with the 

 object of excluding dust-particles. It is saturated with 

 moisture. On taking a stroke of the pump, so as to 

 exhaust the air in the globe, no change is evident ; no 

 condensation has occurred, although the air has been so 

 cooled that the moisture should condense, were it possible. 

 On repeating the operation with the same globe, after 

 admitting dusty air — ordinary air from this room — a 

 slight fog is produced, and, owing to the light behind, 

 a circular rainbow is seen ; a slight shower of rain has 

 taken place. ' There are comparatively few dust particles, 

 because only a little dusty air has been admitted. On 

 again repeating, the fog is denser ; there are more particles 

 on which moisture may condense. 



One point more, and I have done. Work is measured 

 by the distance or height through which a weight can be 

 raised against the force of gravity. The British unit of 

 work is a foot-pound — that is, a pound raised through one 

 foot ; that of the metric system is one gram raised through 

 one centimetre. If a pound be raised through two feet, 

 twice as much work is done as that of raising a pound 

 through one foot, and an amount equal to that of raising 

 two pounds through one foot. The measure of work is 

 therefore the weight multiplied by the distance through 

 which it is raised. When a gas expands against pressure, 

 it does work. The gas may be supposed to be confined i 

 in a vertical tube, and to propel a piston upwards, against 

 the pressure of the atmosphere. If such a tube has a 

 sectional area of one square centimetre, the gas in expand- 

 ing a centimetre up the tube lifts a weight of nearly 

 1000 grams through one centimetre ; for the pressure 

 of the atmosphere on a square centimetre of surface is 

 nearly 1000 granjs— that is, it does 1000 units of work, or 

 ergs. So the work done by a gas in expanding is measured 

 by the change of volume multiplied by the pressure. On 

 the figure, the change of volume is measured horizontally, 

 the change of pressure vertically. Hence the work done 

 is equivalent to the area abcd on the figure. 



If liquid, as it exists at A, change to gas as it exists at 

 15, the substance changes its volume, and may be made 

 to do work. This is familiar in the steam-engine, where 

 work is done by water, expanding to steam and so in- 

 creasing its volume. The pressure does not alter during 

 this change of volume, if sufficient heat be supplied, hence 

 the work done during such a change is given by the 

 rectangular area. 



NO. II 34, VOL. 44] 



Suppose that a man is conveying a trunk up to the 

 first story of a house, he may do it in two (or, perhaps, a 

 greater number of) ways. He may put a ladder up to 

 the drawing-room window, shoulder his trunk, and deposit 

 it directly on the first floor. Or he may go down the 

 area stairs, pass through the kitchen, up the kitchen 

 stairs, up the first flight, up the second flight, and down 

 again to the first story. The end result is the same ; and 

 he does the same amount of work in both cases, so far as 

 conveying the weight to a given height is concerned ; 

 because in going down-stairs he has actually allowed 

 work to be done on him, by the descent of the weight. 



Now, the liquid in expanding to gas begins at a definite 

 volume ; it evaporates gradually to gas without altering 

 pressure, heat being, of course, communicated to it 

 during the change, else it would cool itself ; and it finally 

 ends as gas. It increases its volume by a definite amount 

 at a definite pressure, and so does a definite amount of 

 work ; this work might be utilized in driving an engine. 



But if it pass continuously from liquid to gas, the 

 starting-point and the end point are both the same as 

 before. An equal amount of work has been done. But 

 it has been done by going down the area stair, as it 

 were, and over the round I described before. 



It is clear that a less amount of work has been done on 

 the left-hand side of the figure than was done before ; 

 and a greater amount on the right-hand side ; and if I 

 have made my meaning clear, you will see that as much 

 less has been done on the one side as more has been 

 done on the other— that is, that the area of the figure 

 BEH must be equal to that of the figure afh. Dr. 

 Young and I have tried this experimentally — that is, by 

 measuring the calculated areas ; and we found them to be 

 equal. 



This can be shown to you easily by a simple device— 

 namely, taking them out and weighing them. As this 

 diagram is an exact representation of the results of our 

 experiments with ether, the device can be put in practice. 

 We can detach these areas which are cut out in tin, and 

 place one in each of this pair of scales, and they balance. 

 The fact that a number of areas thus measured gave the 

 theoretical results of itself furnishes a strong support of 

 the justice of the conclusions we drew as regards the 

 forms of these curves. 



To attempt to explain the reasons of this behaviour 

 would take more time than can be given to-night ; more- 

 over, to tell the truth, we do not know them. But we 

 have at least partial knowledge ; and we may hope that 

 investigations at present being carried out by Prof. Tait 

 may give us a clear idea of the nature of the matter, and 

 of the forces which act on it, and with which it acts, 

 during the continuous change from gas to liquid. 



EXPERIMENTAL RESEARCHES ON 

 MECHANICAL FLIGHT. 



THE following is a translation of a communication 

 made by Prof. S. P. Langley to the Paris Academy 

 of Sciences on July 13 ; — 



I have been carrying out some researches intimately 

 connected with the subject of mechanical flight, the 

 results of which appear to me to be worthy of attention. 

 They will be published shortly in detail in a memoir. 

 Meanwhile I wish to state the principal conclusions 

 arrived at. 



In this memoir I do not pretend to develop an art of 

 mechanical flight ; but I demonstrate that, with motors 

 having the same weights as those actually constructed, 

 we possess at present the necessary force for sustaining, 

 with very rapid motion, heavy bodies in the air ; for 

 example, inclined planes more than a thousand times 

 denser than the medium in which they move. '. 



Further, from the point of view of these experiments and. 



