SCIENCE 



NEW YORK, JUNE 19, 1891. 



LOCUSTS IN ALGERIA. 



In his last report on Algerian agriculture, Sir Lambert Playfair 

 remarlis on the spread of locusts from the eastern part of the 

 province, to which they had hitherto for the most part confined 

 their ravages, to the central regions. Until the eminent ento- 

 mologist D'Herculais studied the matter carefully, no specific dis- 

 tinction among the locusts was recognized. He has now shown, 

 according to the London Times, that there are two distinct species, 

 belonging to separate genera, each of wliich has very marked pe- 

 culiarities. These are the best known of the Biblical species, 

 Acridium peregriniim, and the Strauronotus maroccanus. Their 

 habits are quite different, the former generally arriving suddenly 

 about April or May, in immense flights, and devastating the green 

 crops. The females penetrate deeply into the moist earth, and 

 deposit their eggs, from eighty to ninety in number, inclosed in a 

 cocoon. Two months afterwards the young locusts or crickets 

 are hatched. They grow rapidly, get their wings in forty-five 

 da,\s, and then continue their cai-eer of devastation far in ad- 

 vance. 



The other species appear in a winged state in July and August. 

 They also ravage what green exists at that season, and the females 

 deposit their eggs at a much less depth than the others, generally 

 on rocky ground. The cocoons do not contain mOre than thirty 

 or forty eggs, and they remain without being hatched till the 

 spring of the following year. The first species finds in central 

 Africa the most favorable circunistances for its development; the 

 second, in more temperate countries, such as the Mediterranean 

 region, and even the Caucasus, Crimea, and Asia Minor. It is 

 the latter that has ravaged Algeria during the last few years, but 

 about the middle of December last the arrival of flights of the 

 Acridium was reported from several of the oases of the extreme 

 south. Fortunately man is not the only enemy of the locust. 

 Starlings and larks feed eagerly on the eggs. Wagon-loads of 

 .these bu-ds used constantly to be sent to the French market, but 

 now the killing of them has been prohibited in the province of 

 ■Constantine. The larvae of the Bombyx eantharis and other in- 

 ■sects also get into the cocoons, and often kill from ten to fifty per 

 cent of the eggs, while minute cryptogamic organisms destroy 

 many more. 



The best method of contending against the locust has been very 

 •carefully studied. Much has been accomplished by ploughing the 

 ground deeply as soon as possible after the eggs have been laid, 

 so as to bring them to the surface, and thus allow them to become 

 an easy prey to birds and insects. The collection and destruction 

 of the cocoons by manual labor is less sure and more costly, but 

 it has the advantage of affording employment to Arabs, who have 

 ,been reduced to great misery by the destruction of their crops. 

 The statistics of locusts thus destroyed is startling. It has been 

 •calculated that between August and December, 1888, the enor- 

 mous quantity of 8,000 cubic metres of cocoons were collected and 

 destroyed, and that these contained 200,000,000,000 eggs. After 

 ■•the insects were hatched, 1,200,000,000,000 crickets were killed, 

 and it was the excess beyond these iigures that invaded the land. 



It is now admitted that the most efficacious means of waging 

 war on the locusts is to concentrate all available resources on the 

 destruction of the young. They remain quite stationary during 

 five or six days after being hatched, and thus time is allowed for 

 their destruction. The Arabs employ very primitive means: they 

 jump among them, treading and crushing them under foot, beat- 

 ing about in every direction with branches of liroom and oleander, 

 and lighting immense tires all over the place, with alfa grass, or 

 .any dry brushwood that may be available. The most practical 



method is the use of screens similar to those employed in Cyprus. 

 These are bands of cotton stuff, twenty to twenty-five metres in 

 length, on which are sewn strips of American wax-cloth. The 

 young crickets climb up the former, but when they arrive at the 

 latter they can find no foothold, and tumble back into ditches 

 prepared for their reception, along which sheets of zinc are placed 

 to prevent their egreSs. As soon as the ditches are filled, the in- 

 sects are covered over with earth and the screens advanced. 

 During last season the material provided in Algeria, but which 

 was altogether insufficient, was 6,000 screens, each 50 metres 

 long; 100,000 oak pickets; 6,000 steel hammers; 450,000 metres 

 of cord ; and 60.000 sheets of zmo. 



STEAM-JACKET EFFICIENCY. 



In a paper on "Maximum Steim-Jacket Efficiency," con- 

 tributed to the Journal of the Franklin Institute, Professor Robert 

 H. Thurston says the fact is sufficiently well known that the steam- 

 jacket, as employed on the steam engine, of whatever form and 

 arrangement, is intrinsically a wasteful element, and that its use 

 only gives, in certain cases, an economical advantage by its re- 

 pression of wastes of larger magnitude. It checks a serious una- 

 voidable waste, more or less completely, bj' a process vvhich as 

 inevitably involves a waste which is commonly, but, perhaps, not 

 invariably, a lesser one. The ideal steam engine, such as is treated 

 of in the purely thermodynamic study of the steam engine, has a 

 lower efficiency with, than it has without, a jacket. This is readily 

 seen from illustrations computed and checked by Messrs. Hitch- 

 cock and Mount, at the suggestion of Professor Thurston, and 

 published in his paper; and it is sufficiently evident, d priori, from 

 the consideration that the unjacketed engine receives all its steam 

 at a maximum temperature, expands it adiabatically to a certain 

 terminal temperature, and then exhausts it ; while the jacketed 

 receives a part of its heat at intermediate temperatures, expands 

 the fluid non-adiabatically, and finally rejects it at the terminal 

 temperature, with a lower mean range of expansion. In other 

 words, the jacketed engine departs furthest from the principles of 

 economical operations first enunciated by Carnot : " All heat 

 should be received at maximum temperature; expansion should 

 be perfectly adiabatic, and should continue to the minimum tem- 

 perature and pressure, and all should be rejected as nearly as 

 possible at that minimum."' Thus, •• theoretically," if the use of 

 that much-abused term may be .permitted in this sense, the un- 

 jacketed engine is more efficient than the jacketed engine. 

 "Practically," however, the reverse is usually, though probably 

 not always, the case, and the use of the jacket is often found to be 

 productive of a real, and sometimes of large, economy. It is thus 

 obvious that the advantages of the employment of the jacket 

 come of those conditions which distinguish so markedly the real 

 . from the ideal case in steam-engine economy ; tho^e which make 

 the " theory of the real engine." as the writer has called it, essen- 

 tially different, in important respects, from the " theory of the 

 ideal engine." In 1886 a "research committee" was appointed 

 by the British Institution of Mechanical Engineers, to investigate 

 the subject of the steam jacket. A very unusually complete set 

 of data, pertaining to trials made with a view to determine the 

 efficiency produced by application of the jacket, was secured. 

 From computations based on these data, performed with great care, 

 the computers checking the figures and the results, there can be 

 no doubt of the existence of a maximum in the value of the steam 

 jacket, the ratios of expansion being varied, and it is probably 

 fairly to be assumed that it may be found in all cases. In the first 

 case, that of the simple non-condensing Corliss engine, the heads 

 unjacketed, the use of the jacket reduced the cylinder wastes from 

 about twenty-five per cent of the i'ieal consumpt on of steam and 



