92 



NATURE 



{May 23, 1878 



and a markedly sterile desert, this strip of territory seem- 

 ingly wanting in none of Nature's riches save flowing 

 rivers, has been conquered successively by the Romans, the 

 Vandals, the Byzantines, the Greeks, and the Arabs. All 

 these several possessors came and conquered and settled 

 on these lands ; but the first four civilisations died away, 

 and the last is disappearing, in at least the large central 

 portion of this district known as Algeria, and now under 

 French rule. Who can tell whether this new phase will 

 have any more vitality than the rest ? — for the native tribes 

 seem to be as unreclaimable as their own Sahara. 



The many architectural ruins scattered over this dis- 

 trict still attest the greatness of her conquerors, and the 

 visitor to any of the provinces of Algeria, more especially 

 to Algiers, will be astonished at the size and grandeur of 

 these remains. In 1765 the traveller Bruce, as he tells us 

 in an autobiography, was told by my Lord Halifax, " that 

 the way to rise in the king's favour was by enterprise 

 and discovery ; that all Africa, though just at our door, 

 was yet unexplored ; that every page of Dr. Shaw, a 

 writer of undoubted merit, spoke of some magnificent 

 ruins which he had seen in the kingdoms of Tunis and 

 Algiers, and that now was the age to recover these 

 remains and add them to the king's collection." With 

 this suggestion Bruce was offered the post of consul at 

 Algiers, with a good salary. Bruce at once put aside for the 

 moment all thoughts of the fountains of the Nile, "as 

 involving an enterprise above the powers of an untried 

 ordinary man," and setting out for Italy, he passed through 

 France, and was carried in H.B.M. frigate Montreal 

 from Naples to Algiers. The story of Bruce's life is yet 

 to be written ; probably no traveller has ever had to con- 

 tend against a greater amount of ill-deserved obloquy. 

 His account of his travels, we know, was received with 

 the greatest incredulity, and yet there are very few of his 

 statements that have not, since he published them, been 

 abundantly confirmed. It would seem but an act of 

 justice that we should, in the light of modern discovery, 

 have a new edition of Bruce's Travels thoroughly well 

 annotated, and we can think of no one so well qualified 

 for this task as the author of this volume, which gives us 

 an account of how Col. Playfair came to travel in the 

 footsteps of this great father of African travel. 



Bruce, we have seen, was British Consul- General at 

 Algiers in 1765, and he received this appointment to enable 

 him to examine and describe the many fine ruins said so 

 truly by Ur. Shaw to be scattered over Tunis and Algeria. 

 An account of these travels, with detailed descriptions 

 and drawings of these ruins, was prepared by I5ruce, 

 with the intention of publishing them ; but it is probable 

 that the manner in which the simple narrative of his 

 travels was received by the public had the effect of 

 making him abandon this idea. 



We must refer the reader to this volume for information 

 as to how Col. Playfair, who now occupies Bruce's place 

 as H.M. Consul-General at Algiers, discovered Bruce's 

 manuscripts and drawings in the library of Lord Thurlow, 

 whose wife is the great-great-granddaughter of the tra- 

 veller as well as heiress of Kinnaird. As a result of this 

 discovery. Col. Playfair determined to follow Bruce's foot- 

 steps in Tunis and Algeria, to visit every ruin which he 

 had illustrated, and so to plan his route as to include all 

 that was most picturesque and instructive in a country 

 that is even yet hardly at all known to the modern traveller ; 

 and well he does all this in the sumptuous quarto volume 

 before us. 



This volume will form a lasting monument to the 

 memoiy of Bruce, and some five-and-twenty of the large 

 quarto illustrations, being facsimiles from Bruce's draw- 

 ings, will serve to show how accurate as an architec- 

 tural draughtsman he was, and how independent he 

 might — had it not been for the fashion of the times — 

 have been of the "adornments" of his Italian artist. 



■ E. P. Wright 



LETTERS TO THE EDITOR 



yrhe Editor does not hold himself responsible for opinions expressea 

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On the Availability of Normal-Temperature Heat- 

 Energy 



1. I WAS interested to see the letter of Prof. A. S. Herschel, in 

 Nature, vol. xviii. p. 39, referring to my papers on the subject 

 of the derivation of work from normal-temperature heat 

 (Nature, vol. xvii. p. 31, and vol. xvii. p. 20). There is one 

 qualification (not affecting the reasoning in my papers) I should 

 like to make here, viz., the case there dealt with does not appear 

 necessarily to be out of harmony with what is termed the "second 

 law of thermodynamics," though it may be questioned whether it 

 quite harmonises with certain modes of stating the law. As the 

 Jacts are of course the most important, it may perhaps be well 

 just briefly to recapitulate here under what conditions normal- 

 temi^erature heat can be converted into iwork, as affecting the 

 problem dealt with. 



2. Firstly, we observe that the diffusion of matter (gases) 

 enables us to derive work solely at the expense of normal-tem- 

 perature heat {i.e., without an artificially-produced fall of tem- 

 perature or a source and a refrigerator) ; and through this pro- 

 cess a mass of gas may (by unchanged total volume) acquire a 

 capacity for work without the performance of work. I say that 

 the work is derived solely at the expense of normal temperature 

 heat, because, although the mixing or diffusion is a necessary 

 accident to the derivation of the work, it is in no way the source 

 of the work, or the heat lost is the exact mechanical equivalent 

 of the work derived, and the diffusion has nothing to do with 

 furnishing the work. Still the system, after diffusion, is evidently 

 not restored to its original state in all respects, except the trans- 

 ference of heat, although the passage of the system from its 



original to its final state did not furnish any of the work. To 

 violate the second law of thermodynamics, the system would re- 

 quire to be restored to its original state, and if a method could 

 be discovered for doing this, clearly no work would be theoreti- 

 cally required merely to effect this result (since the total volume 

 of the gases is unchanged by the mixing). 



3. It would seem scarcely to be brought into sufficient promi- 

 nence in connection with the statement of the "second law of 

 thermodynamics," how far it is possible (without a source or 

 refrigerator) to convert normal-temperature heat into work, which 

 is really the practical point ; for there are many cases where it is 

 of no consequence to us whether the matter from which the heat 

 is derived is mixed or not. The great point is to derive the heat 

 or convert it into useful mechanical energy. This is what it was 

 my main object to treat of in the two papers referred to. 



4. No doubt in nature normal-temperature heat is thus largely 

 utilised or converted into work. For in the functions of plants 

 and animals, two gases of different molecular weights, oxygen 

 and carbonic acid, are largely concerned, and animal and vegetable 

 structures are notably /i^r^MJ, so that no doubt normal-tempera- 

 ture heat may be converted into work through diffusion in this 

 way — i.e., by the different rates of diffusion of the two gases 

 across the porous tissues. Indeed this principle might con- 

 ceivably have more to do with animal and vegetable functions 

 than is imagined. There are also mixtures of gases used in 

 industrial operations, such as for explosion in gas-engines, &c. 

 No doubt in such cases before exploding the gases, normal-tem- 

 perature heat might be converted into work through diffusion 

 (by means of a porous diaphragm connected with suitable 

 machinery, in the manner roughly sketched), and the utility of 

 the process would depend simply on the quantity of gas at 

 disposal. 



5. In regard to the means afforded by a porous diaphragm for 

 (as it were) manipulating molecules and sifting them according 

 to their velocities ; no doubt this (as Prof. A. S. Herschel 

 remarks) somewhat resembles the functions performed by the 

 ideal being or "sprite" described by Prof. Maxwell in his 

 "Theory of Heat," but it does not quite attain that result, for 

 the diaphragm can only effect an unequal distribution of energy 

 combined tvith an unequal distribution of matter, so that the 



