SCIENCE. 



[Vol. XIV. No. 345 



— Harper & Brothers have just ready " A History of the Kansas 

 Crusade : its Friends and its Foes," by Eli Thayer, who planned 

 and organized the movement by which Kansas was made a free 

 State, with an introduction by Edward Everett Hale, a fellow- 

 worker with Mr. Thayer in the emigration cause ; and " Man and 

 His Maladies," a popular handbook of physiology and domestic 

 medicine, by A. E. Bndger. 



LETTERS TO THE EDITOR. 



*^*Correspondetits are requested to be as brie/ as possible. The -writer's name is 

 in alt cases required as proof of f;ood faith. 



The editor wilt be glad to publish any queries consonant with the character of 

 the journal. 



Twenty copies of the number containing his cominunicaiion will be furnished 

 free to any correspondent on request. 



The Law of Population in the United States. 



Pursuing the investigation of the law of population, we come 

 to a question of importance in an economical and ethnographic 

 view. What is to be the relative progress in numbers of the Cau- 

 casian and the African races here ? 



The late revolution in Hayti has led to the publication in the 

 daily press of America of so many concise abstracts from the his- 

 tory of that African republic as to make it familiar to all who in- 

 terest themselves in these matters. It is to be regretted that the 

 progress of regulated liberty in that island has not, in a century, 

 been greater. It is a prevailing belief that with us the African in- 

 creases faster than the Caucasian. The figures of the census 

 during a hundred years do not confirm this opinion. We find that 

 while the whites since 1790 have increased on the average in each 

 decade by 33.46 per cent, the blacks have gained in the same time 

 only 26,81 per cent. 



Applying these rates to the present numbers we may forecast 

 \\\^ possible, \i xvoX. the probable, population, during the next cen- 

 tury as follows : — 



Year. Total popula 



i8go 67,240,0c 



igoo 89,738,0c 



1910 119,650,01 



1920 I59,890,0( 



1930 213.320,0' 



1940 284,697,01 



19^0 379,960,0. 



i960 507,090,0 



1970 676,760,01 



African descent. 



903,; 



iggo 1206,400,0 



..33,252,000 



.42,163,000 12 " 



■53,463.000 

 ..67,790,000 

 ■■85,957,000 14 " 



The reader can draw his own inferences from these significant 

 figures. We only say that in 1940 and thereafter this country will 

 not be able to offer free space and citizenship and suffrage for the 

 surplus overflowing of China, to a race which does not assimilate 

 with us, and which is pagan ; and that it is time to discontinue the 

 complaint that the Chinese exclusion act was mere demagogism. 

 In the light of these figures, it was the highest statesmanship. The 

 importation of native Africans ceased by the Constitution in 1808, 

 though it is alleged that a few fanatics imported cargoes later. 

 But practically the forced importation ceased then. There never 

 has been any voluntary immigration from Africa. 



Both Malthus, in 1794, and Alison, in 1840, held that the popu- 

 lation of the United States after 1640 doubled every twenty-three 

 and a half years. This rate has continued to 1890, for two hun- 

 dred and fifty years. M. C. Meigs. 



Washington, D.C, Sept. 2. 



INDUSTRIAL NOTES. 

 Comparison of different Street Car Systems. 



Being continually requested by street railroad men to furnish 

 them with a statement of the cost of equipment and operation of a 

 road by means of storage battery traction, and also how the cost 

 of this method of traction will compare with other systems, the 

 Julien Electric Traction Company of this city have made a com- 

 parison of the four methods available to-day for street-car propul- 

 sion in large cities, — horses, storage batteries, electric conduit, and 

 cable. The estimates and comparisons, it is claimed, have been 

 carefully prepared, and special attention has been given to obtain 

 good authority for statements, mostly from roads having the differ- 

 ■ent systems in actual operation. The estimates are based on a 

 medium-sized road running on the headway generally employed in 

 cities, trying, as far as possible, to cover roads operating under 

 •such different circumstances as are found in different localities. 



The company mainly aim to treat the subject as applied to 

 cities. They have not included figures on the overhead system as 

 they consider them barred from operating in that field, owing to 

 vthe necessity of the presence of overhead electric conductors, and 

 the growing sentiment in all communities against the erection of 

 ipoles. As regards the Julien system, the figures show the results 

 ■of two years' experience on the Fourth and Madison Avenue line 

 lin this city. 



The estimates are based on a road six miles long, double track, 

 ■operating sixty cars, running eight miles an hour by mechanical, 

 .and six miles an hour by animal traction, running on one and one- 

 ihalf minutes headway, and eighty-four miles a day in the former, 

 .and on two minutes headway, sixty miles a day, in the latter case, 

 allowing nine horses to a car. The item of building and land is 

 not included, as they differ so widely in different cities and localities. 



According to the figures given, the cost of constructing and 

 equipping such a road, on the Julien storage battery system, would 

 be S491.500. c. if the current were taken from a central lighting 

 station, $419,000; the same road constructed and equipped for a 

 ihorse railroad, §229,620; as a cable road, $1,076,000; as a conduit 

 • electric road, $762,000. The annual running expenses under the 

 .different systems would be as follows. Julien system, eighty-four 

 .miles a day per car, with electric plant, $99,206, being $4.52 per 

 ■car-day, or .053 of a cent per car-mile ; same system, using current 

 irom a lighting station, $113,330, being $5.17 per car-day, or .061 



of a cent per car-mile ; horse traction, sixty miles a day per car, 

 $129,562.20, being $5.91 per car-day, or .098 of a cent per car- 

 mile ; cable road, eighty-four miles a day per car, $163,712.50, be- 

 ing $7.47 per car-day, or .089 of a cent per car-mile ; electric con- 

 duit system, eighty-four miles a day per car, $111,157.50, being 

 $5.07 per car-day, or .06 of a cent per car-mile. 



Carhart-Clark Standard Cells. 



In last week's Science appeared an abstract of a paper on an 

 improved form of Clark standard cell read before the American 

 Association for the Advancement of Science by Professor H. S. 

 Carhart of the University of Michigan, vice-president of the PhjB- 

 ical Section of the association. We add a few points relating to 

 the special features of this cell and its mounting for commercial 

 and scientific purposes, as sold by James W. Queen & Co., Phila- 

 delphia, who have the exclustve handling and sale of it. 



"This cell embodies several new and important features, chief 

 among which are its low temperature coefficient and safety in 

 transportation. These features are secured by the methods of 

 Professor Carhart, devised after a series of investigations extend- 

 ing over nearly three years. The change of electromotive force 

 produced by a temperature change of a few degrees is practically 

 negligible except in scientific work of the greatest accuracy. The 

 coefficient is only 0.038 per cent per degree C. This is somewhat 

 less than one-half the coefficient of Lord Rayleigh's form, for 

 which he found a value ranging from 0.077 pe"^ cent to 0.082 per 

 cent per degree. Almost absolute safety in transportation is se- 

 cured by confining the mercury to the bottom of the cell, thus pre- 

 cluding the possibility of its reaching the zinc and short-circuiting 

 the cell, no matter how violently it may be shaken. This process 

 presents the additional advantage of increasing the electromotive 

 force about 0.35 per cent above the old form, and of preventing lo- 

 cal action, by which very serious changes took place in the old 

 form of the cell on open circuit. Greater uniformity and constancy, 

 it is believed, result from this method of making a cell. 



Another well-marked characteristic of these new cells is their 

 remarkable uniformity. This is due to great care in the prepara- 

 tion of the salts and standard solutions, and to the absolute clean- 

 liness observed in every part of the cell. In the Clark cell, as 

 made by Lord Rayleigh, the mercury salt always turned from its 

 normal white to a canary yellow on mixing with the zinc sulphate, 

 a change probably due to the presence of mercuric salt. In this 



