PHYSICS. 435 



reduction; a careful account by M. Marek of the excellent normal ba- 

 rometer and catbetometer in use at tbe bureau, and a description of the 

 methods of calibrating the thermometers used during the weighings; 

 and illustrations of the specific gravity apparatus, and of the method 

 of Stas for clearing the surfaces of metals by a jet of alcohol vapor. 

 Besides these memoirs, the International Committee publishes reports 

 to the Govern ments represented, of which the seventh appeared in 1884. 

 Great Britain has now a representative on this committee. {Nature^ 

 April, xxis, 529 ; October, 1884, xxx, 612. Science^ iii, 305, March, 

 1884.) 



Herschel has given a short method of converting yards into meters, 

 and the converse, based upon the ratio of the meter to the yard ob- 

 tained by W. A. Rogers, in his comparisons of the " metre des archives" 

 with the standard yard. This ratio, which is 39.37027 : 36, agrees with the 

 simpler one, 35: 32, very closely, the error being only 1 part in 8,000. Her- 

 schel's rule is as follows : To convert yards into meters, subtract one- 

 tenth and add oneseventh^f that tenth. If great accuracy be required, 

 add one oue-hundred-and-tbirtieth of that seventh. The result is ac- 

 curate to the sixth decimal place. To convert meters into yards, add 

 one-tenth, subtract one-sixteenth of that tenth, two one-hundredths of 

 that sixteenth, and one-twentieth of that two one-hundredths. The error 

 is less than 5 inches in the kilometer, or less than 2 decimeters in the 

 mile. {Wature, July, 1884, xxx, 312.) 



Johnstone Stouey has called attention to the fact that Clarke's deter- 

 mination of the yard in metrical units dititers from Kater's by more than 

 the diiierence between Clarke's and the simple value 914.4 millimeters 

 for the yard. Hence the yard may be assumed to have this value, with 

 an error of less than a fifth metret (lO"'^) in the meter. So the pound 

 may be assumed .as 453.6 grams, with an error of one-fourth of a grain 

 avoirdupois, on the authority of Professor JMiller; and the gallon may 

 be considered as 4,544 cubic centimeters, with an error of less than one 

 cubic centimeter in 10 liters. {Nature, January, 1884, xxix, 278.) 



Rowland has proposed to cous,truct scales with his ruling-engine, which 

 will enable the physicist at any time, by purely optical means, and 

 without knowing the coetficieut of expansion of the metal or its temper- 

 ature, to obtain the value of the length of the scale in terms of the wave- 

 length of any given ray of light. These scales are simply to be straight 

 pieces of speculum metal, ruled with lines just like an ordinary grating, 

 except that the length of the lines is to be only about one centimeter, 

 every one-hundredth line being somewhat longer than its neighbors ; the 

 whole ruled slip is to be one decimeter in length. From the manner of 

 ruling it will be easy to count the whole number of lines in the length 

 of the strip, and then, by a simple use of the scale as a grating in a 

 suitable spectrometer, the whole length may be immediately found at 

 any time, in terms of any specified wave-length of light. {Science, Sep- 

 tember, 1884, IV, 296; WaUtre, October, 1884. XXX, 596.) 



