Decembek 5, 1902.] 



SCIENCE. 



883 



Along with these laboratory studies in 

 Troy Professor Rood conducted a series 

 of out-door investigations which were pub- 

 lished in 1860 under the title 'Experi- 

 ments on the Forms of Elongated Pro- 

 jectiles.' He was fond of rifle practice, 

 and in Troy at that time there was a 

 rifle manufactory where probably the best 

 weapons of this kind in America were 

 made. He devised a special form of 

 ballistic penduhim for measuring velocity, 

 studied the relation between accuracy of 

 flight and the rate of rotation of the pro- 

 jectile on its longitudinal axis, and investi- 

 gated penetrative power as related to the 

 form of the projectile, its initial velocity 

 and the position of its center of gravity. 

 Comparing his results with those attained 

 in England and on the continent of Eu- 

 rope, he demonstrated the marked superi- 

 ority of American rifled guns. The coun- 

 try was on the eve of civil war, and the 

 investigation was of much more than theo- 

 retic interest. 



While in Troy Professor Rood was active 

 as an amateur photographer. In 1861 and 

 1862 he published papers ' On the Practical 

 Application of Photography to the Micro- 

 scope,' 'On the Investigation of Micro- 

 scopic Forms by Means of the Images 

 which they Furnish of External Objects,' 

 and 'On the Study of the Electric Spark 

 by the Aid of Photography.' About the 

 same time he was the pioneer in the suc- 

 cessful construction of fluid prisms of 

 highly dispersive power for the study of 

 the spectrum, attaining a degree of accu- 

 rate deflnition far in advance of what had 

 previously been accomplished with such 

 prisms. 



Professor Rood's demonstrated ability as 

 an experimentalist and the reputation he 

 had rapidly made by his researches were 

 what determined his call to Columbia Col- 

 lege in 1864 and his election to member- 

 ship in the National Academy of Sciences 



during the same year. In New York he 

 developed a long-continued research on the 

 use of the revolving disk as a means of 

 measuring very small intervals of time, 

 still continuing his studies on the spec- 

 trum, and specializing on the quantitative 

 analysis of the phenomena of color mix- 

 ture and color contrast. But this did not 

 Ijrevent temporary excursions into other 

 fields. In 1874 he published an 'Optical 

 Method of Studying the Vibrations of 

 Solid Bodies,' and during the same year 

 he made quite an elaborate research 'On 

 the Application of the Horizontal Pendu- 

 lum to the Measurement of Minute Changes 

 in the Dimensions of Solid Bodies.' The 

 exactitude of this measurement is indicated 

 by the statement that the probable error 

 of a single measurement was reduced to 

 about one twenty-millionth of an English 

 inch, or roughly one three-hundredth of a 

 wave-length of violet light. 



The use of the revolving disk was spe- 

 cially applied to observations of the dura- 

 tion and multiple character of flashes of 

 lightning and of disruptive discharges be- 

 tween the electrodes of induction coils and 

 influence machines. Other investigators 

 had estimated the duration to be, in some 

 cases, as little as one millionth of a second. 

 It was shown by Rood that this was far too 

 small. The actual range of variation is 

 of course great, but his experiments indi- 

 cated that, for a Leyden jar connected with 

 an induction coil, an average value was 

 about one flve-hundredth of a second. 



In 1880 and 1881 Professor Rood de- 

 voted much attention to the study of 

 vacuum pumps, his aim being not to in- 

 crease their commercial efficiency, but to 

 ascertain the limit of perfection attainable, 

 even though with such expense of time as 

 to interfere with ready availability. He 

 modified and so improved the Sprengel 

 mercury pump as to secure a vacuum much 

 more nearly perfect than had been secured 



