.36 



SCIENCE. 



[Vol. X. No. 232 



all the components of motion ; which must give us the true in- 

 tensity in just the sense that this term is employed by Mr. Hayden 

 .and myself. Its graphic representation will be the curves we have 

 given and no other. 



Professor Le Conte remarks : " We have assumed all along that 

 the intensity or excursion of the earth-particle, or the height or 

 amplitude of the wave, varies inversely as the square of the radius 

 of the agitated sphere. The authors as well as other writers 

 assume this law." Here he evidently misapprehends. It is indeed 

 assumed that the intensity varies inversely as the square of the dis- 

 tance, but the amplitude varies (subject to later qualification) in a 

 simple (not duplicate) inverse ratio with the distance. The intensity 

 for a given wave-length is proportional to the square of the ampli- 

 tude ; for, by Hooke's law {ut tensio sic vis), the time of vibration 

 of a particle in an elastic wave of given wave-length is uniform 

 ■whatever the amplitude. Hence the mean velocity of the particle is 

 simply proportional to the length of its path, i.e., to the amplitude. 

 But its energy is proportional to the square of its velocity, ergo, to 

 .the square of its amplitude. Hence, too, the amplitude must be in- 

 versely proportional to the radius of the spherical wave, provided 

 no energy is dissipated in transmission. If, then, the amplitude at 

 Charleston were four inches, at a distance of a thousand miles it 

 would, without dissipation, amount to about two millimetres, — a 

 ■well-marked tremor. 



Professor Le Conte's suggestion that the law of variation of in- 

 tensity with distance may be affected by reflection back into the 

 earth from the surface is, so far as I am aware, a novel one. That 

 there must be some energy so reflected seems undisputable. But 

 .the portion so reflected would constitute a new wave, or series of 

 new waves, independent of those already in progress. It would thus 

 add to the number of waves without affecting the energy of those 

 -already in progress, except at points of coincidence or interference. 



Seebach's method of finding the depth was objected to, because 

 it requires a degree of accuracy much beyond the highest we can 

 hope to attain. The speed of an earthquake-wave is enormous 

 (the time-observations obtained for the Sonora earthquake give a 

 Trery high wave-speed ; they are not as yet fully examined and dis- 

 cussed, but the preliminary examination indicates a speed about the 

 same as that obtained in the Charleston earthquake), the space- 

 intervals at which the time-records must be made must be short, 

 and the time-intervals correspondingly so. The data really needed 

 are differences in these time-inten'als ; and these differences would 

 most certainly be much smaller than the probable errors of obser- 

 vation. C. E. DUTTON. 



Washington. July 9. 



The Freezing-Point of Sea-Water, and the Melting-Point of 

 Sea-Water Ice. 



The difference existing between the result from my determina- 

 tion of the freezing-point of sea-water (^Science, ix. No. 228), and 

 the accepted one as value for the same of 28°. 8 F., seems to be 

 inexplicable, unless we can assume that in the methods followed 

 ior its determination a wrong interpretation has been put on one of 

 the results. 



There can be no doubt, that, if the temperature of a body of sea- 

 water is lowered till conglaciation takes places under slight 

 agitation of the water, the temperature then existing at its surface 

 will be that of its freezing-point. 



On the other hand, it seems probable, that, when the determina- 

 tion of the freezing-point is made by means of an admixture of 

 sea-water and its ice in thermic equilibrium, we have reached a 

 condition that would be better described as the melting-point of 

 sea-water ice. 



Could we assume that in the change from the liquid to the solid 

 form, in freezing, all the saline particles were taken up without 

 chemical changes, it would be reasonable to suppose that the melt- 

 ing and freezing points would coincide ; but if, on the other hand, 

 we assume that in this conversion the entire saline particles have 

 been expelled from the solid, we must conclude that part of the 

 heat was expended in expelling these particles, for we may not 

 imagine Miy work performed without a corresponding absorption 

 of energy. We will have, in this imaginary case, essentially fresh- 

 water ice ; and, if we were determining the freezing-point of sea- 



water with ice so constituted, thermic equilibrium would be ob- 

 tained at a temperature of 33°, which we should erroneously call 

 the freezing-point of sea-water. 



Granting the accuracy of these two suppositions, it seems certain 

 that in the case when freezing takes place to the exclusion of four- 

 fifths of the saline particles, as is the case with sea-water, thermic 

 equilibrium will exist between sea-water and its ice at a tempera- 

 ture intermediate between its freezing-point {26°. y F.) and that of 

 melting ice (32''.o F.), and experiment has proved this temperature 

 to be 28°.8 F. 



I would therefore predict, that, in the case where a liquid is con- 

 verted into a solid by freezing, the temperature of the freezing- 

 point of the liquid will be equal to that of the melting-point of the 

 ice, only in the case or cases where each contains the constituents 

 of the other in the same proportion. W. A. Ashe. 



The Quebec Observatory, July 4. 



Concerning Filth-Diseases. 



Through the heart of the city of Baltimore, flowing southward, 

 runs the bed of the sluggish stream Jones Falls. Eight miles 

 northward, its waters are divided and turned into the city water- 

 supply. Within the city, the stream is confined by handsome stone 

 walls, which form a canal of dimensions twenty feet deep by a 

 hundred feet wide and two and a half miles long — roughly. The 

 canal empties into the Back Basin, a nearly stagnant pool two 

 hundred yards wide by five hundred yards long, — which itself is 

 connected by a short canal with the City Basin and tide water of 

 slight activity. 



In the northern suburbs, and within the city, the Falls and 

 Back Basin receive the drainage from a territory in which dwell 

 eighty thousand souls, roughly estimated, a considerably portion 

 of whom is packed into a lower quarter of the city. They receive 

 that from the Causeway and a part of Fell's Point, — quarters fairly 

 designated slums. 



The sediments in this drainage are precipitated in the lower half- 

 mile of the Falls and in the Back Basin. Here they undergo fer- 

 mentation and decay, at times giving off odors offensive indeed. 

 It is a necessity of the situation that these sediments must be re- 

 moved by dredging ; and with the active officials of the dredging 

 companies, and with their workmen, the writer has been in quite 

 constant communication for nearly three years. These people pass 

 their days stirring about and digging up this fermenting and decay- 

 ing city garbage and mud and sewage. They live in an atmosphere 

 loaded with offensive gases. And what of their health } With 

 singular unanimity they declare that the occupation is a healthy 

 one. Excepting in rare instances a case of nausea and vomiting, 

 which quickly pass away, they have no more sickness than those 

 in other occupations. As a matter of fact, the writer has not in 

 nearly three years heard of any case of zymotic disease among 

 about a hundred men engaged in this dredging. 



The decaying refuse from the slums of a city, deposited in warm 

 and nearly stagnant waters, ought to contain all manner of poison- 

 ous elements, — animal, vegetable, gaseous, or otherwise. Men 

 stirring up and removing such material ought to sicken and die. 

 Curiously enough, they do not — more than men in other occu- 

 pations. 



The writer has no knowledge as to what filth-diseases are, or are 

 not, and has no suggestions to offer. The studies here indicated 

 were made because the field seemed to promise a rich harvest of 

 such diseases, but the promise has not been fulfilled so far. 



Wm. Glenn. 



Baltimore, July 11. 



Queries. 



8. Whooping-Cough in the Cat. — A Liverpool cat is re- 

 ported to have contracted whooping-cough from a boy sick with 

 that disease. For two weeks it had five or six attacks daily of the 

 cough characteristic of that affection. Is this unusual } X. 



9. Banana, Cocoanut, and India-Rubber. — Can any one 

 send us lists of books on the cultivation of the banana, cocoanut, 

 and india-rubber ? J. C. E. 



