April 14, 1893. J 



SCIENCE. 



207 



times as much as the seed, but the dry matter in these plants was 

 from 86 to 130 per cent only of that in the seed planted. 



Details of Weights and Measurements. 



Growth above ground of two plants three weeks after planting. 



ELECTRICAL NOTES. 



Some of the practical results of Dr. Sumpner's work on pho- 

 tometry were alluded to in a previous note. As the Proceedings 

 of the Physical Society are not generally accessible, and most of 

 the abstracts given are rather brief, it may be worth whiletogive 

 a short account of some of the more theoretical results. 



The first is the practical demonstration of the very approxi- 

 mate accuracy of the cosine law of reflection of such substances 

 as white paper, tracing cloth, and white cloth. From this fol- 

 lows the remarkable result, confirmed by experiment, that 

 placing a piece of white paper behind a source of illumination 

 more than doubles the illumination at a point normal to the 

 plane of the paper, while the placing of a mirror in the fame po- 

 sition does not quite double it. The reason of this is at once 

 seen to be the fact that the reflecting power of white paper and 

 the mirror are about the same, but that, of a given amount of 

 light falling on the paper, in consequence of the cosine law, the 

 greater part is reflected normally to its surface, whereas in the 

 case of the mirror, the absorption of the glass is greatest in the 

 case of the light falling perpendicularly to it, and so the greater 

 part of the light is given off in directions which are not normal to 

 the surface. 



In the discussion following, it was pointed out that no known 

 shapeof the roughnesses would lead to the mathematicaldeduction 

 ■of the cosine law, so it is probable that the phenomenon of diffu- 

 sion of light is of a somewhat more complicated nature than is 

 generally supposed. It is to be hoped that the definitions used by 

 Dr. Sumpner will be generally employed in photometric work. 

 They are as follows: 



1. Candle-power. — The candle-power of a lamp is measured by 

 the ratio of the illumination of the light considered, to that of a 

 standard candle, both sources being at the same distance from 

 the object illuminated. 



3. Illumination. — The unit of intensity of illumination is that 

 given by a standard candle at a distance of one foot. 



3. Unit quantity of light. — Unit quantity of light is the quan- 

 tity of light which falls on a surface of one square foot placed at 

 a distance of one foot from a standard candle, and so that a nor- 

 mal drawn to the surface at any point, passes through thesource 

 of light. 



The name candle-Toot is given to the unit quantity of light. 

 From the definition, a source of light, candle-power X, gives 

 out a total quantity of light equal to 4 n- candle- foots. 



4. Brightness. — This definition only applies to solids which 

 hecome sources of illumination, either through incandescence. 



as heated platinum, or through reflection, as paper e.xposed to 

 sunlight, i.e., only to such substances as obey the cosine law. 



A surface has unit brightness when a point at a distance of 

 one foot from a surface of one square foot of the substance, and 

 so placed that a normal drawn from any point of the surface 

 passes through, the point, is illuminated with unit intensity. 



From the definition, it follows that the total quantity of light 

 given off by one square foot of surface of brightness, Xis irX. 



One interesting result, following from the considerations which 

 lead to the last of these definitions, is that given by Dr. Sumpner, 

 as it affords an explanation of snow-blindness. 



The total quantity of light reflected from the snow will neatly 

 equal the amount which falls on it. Therefore, if C be the in- 

 tensity of the illumination of the sun at the surface of the snow, 

 the brightness of the snow at a distance of one foot from it will 

 beC|7r. Therefore, if the observer isstandingsothatthesnow-field 

 subtends a solid angle of 90 degrees, we may easily find that 

 the illumination at the point where his eye is, is neatly C, or that 

 the effect is nearly the same as if he were looking straight at the 

 sun. R. A. F. 



LE riEKS TO THE EDITOR. 



»** Correspondents are requested to be as brief as possible. The writer's name 

 is in all cases required as proof of gbod faith. 



On request in advance^ one hundred copies of the number containing his 

 communication loill be furnished free to any correspondent. 



The editor willbe glad to publish any queries consonant with the character 

 of the journal. 



Early Attempts at Storm-Warning. 



In reading Haweis' "Music and Morals," I found on page 868 

 a statement of interest to meteorologists. Writing of the famous 

 Strassbourg tower, he says: "The second bell, recast in 1774, is 

 named 'The Recall' or 'Storm-Bell.' In past times, when the 

 plain of Alsatia was covered with forests and marsh land, this 

 bell was intended to warn the traveller of the approaching storm- 

 cloud as it was seen driving from the Vosges Mountains towards 

 the plain." 



Probably Kopp, Giinlher, van Bebber, or Hellmann, in their 

 records of antiquarian research, have mentioned this early at- 

 tempt at storm-warnings, but I do not remember having seen 

 anything about it. Fe.^nk Waldo. 



Prlncelon, N.J., April 5. 



Pre-Historic Remains in America. 



In his letter in Science, March, 31, under the above title. Pro- 

 fessor Cyrus Thomas misunderstands the quotation which he 

 makes from my ''American Race." He observes, 'If the set- 

 tlement was at one point by one race, and this race was never 

 influenced by another, it is difficult to imagine in what respect 

 the moulding process acted." Is it ? Plainly the moulding pro- 

 cess acted by modifying the intrusive population to another and 

 a fixed racial type by long subjection to an environment to which 

 previously it had never been exposed. Nothing is better recog- 

 nized than such a process; it is taken for granted by all writers, 

 as, for instance, by Dr. Braislin in the same number oi Science 

 in which Prof. Thomas's letter appears; and why such an objec- 

 tion should be offered to my statement, it is even more " difficult 

 to imagine." 



The general theory advanced by Professor Thomas of a funda- 

 mental difference between the civilizations of the Atlantic and 

 Pacific groups, is one for which I have never found any evi- 

 dence. He must know that the ancient civilization of the Mis- 

 sissippi Valley offers as strong, if not stronger, traits of analogy 

 to that of Mexico and Yucatan than does that of the Haidahs. 

 Consider the designs shown on the engraved shells, so well shown 

 in the beautiful monograph of Holmes, or the copper work of the 

 mounds of Ohio and Georgia ! In view of such evidence, how 

 could Prof. Thomas write, that "no such resemblance to those 

 of the Atlantic slope is observable?" Is he not also aware that 

 both the Nahuatl and Maya languages trace their affinities ex- 

 clusively to the eastern and not to the western v ater-shed of the 

 continent? 



