Feb. lo, 1888.] 



SCIENTIFIC NEWS. 



125 



DIAL FOR CHRONOGRAPHS. 



MR. A. P. TROTTER has invented an ingenious 

 dial, which can be used with stop watches to de- 

 termine the distance of a sound, the space a body falls 

 through, or the rate a train is travelling at. The outer- 

 most or first circle on the dial, shown in our illustration, 

 is graduated into seconds and fifths of seconds, as is usual 

 in chronograph dials. The second circle is divided, so as 

 to show the space in feet traversed by a falling body in 

 the interval between the moment of release and the 

 instant of contact with the ground. The third circle also 

 shows this distance when the time is noted between the 

 release of the object and the sound of its impact with the 

 ground. These two circles are only graduated as far as 

 1,000 feet, as at the end of eight or nine seconds the 

 velocity acquired by a falling body becomes so great that 

 the air resistance sensibly affects the time occupied in 

 falling a certain distance. The fourth circle shows the 

 number of miles per hour a train is travelling, if the 

 second-hand be started when entering upon, and stopped 



position shown in our illustration, it can be made to 

 sound by tapping it lightly, when the button will dance 



at the end of, a quarter of a mile. The graduations on the 

 fifth or innermost circle show the distance in miles cor- 

 responding to the time between the flash of a gun or of 

 lightning and the arrival of the sound. This dial should 

 prove a valuable addition to stop watches and chrono- 

 graphs. 



«^>t^^5<--<. 



SOUND WAVES. 



IF a stone be thrown in a quiet pool of water, circular 

 waves are seen to start from the spot, which open 

 out as they travel over the surface of the water. A blow 

 on a table with a stick is supposed to cause similar 

 waves in the air, and these vibrations falling upon the 

 drum of the ear produce the sensation to which the 

 name sound has been given. 



In the case of a violin-string, the oscillations producing 

 the sound are visible to the eye ; the vibrations of a 

 wine-glass are not quite so perceptible, but, as has been 

 pointed out in La Nature, they may be made manifest by 

 fastening a boot button to the stem by means of a piece 

 of cotton of such a length that the button nearly reaches 

 the rim of the glass. If the glass is then held in the 



to and fro, thus showing plainly that the surface on which 

 it is resting is in a state of motion. 



RATIONAL DEVELOPMENT OF PHOTO- 

 GRAPHY. 



TO understand the rationale of the scientific develop- 

 ment of a photographic dry-plate does not entail 

 much mental labour or abiUty, and yet, strange to say, 

 many amateurs are quite content to work by a purely 

 " rule of thumb " process. The writers of many of our 

 photographic primers are much to blame in the matter, 

 but the greatest offenders of all are the dry-plate manu- 

 facturers themselves, who publish the formulae bestsuited 

 to their plates in such a random, unscientific manner, 

 rendering necessary a series of complicated calculations 

 before the desired information can be ascertained. Eight 

 amateurs out of ten will not take the trouble, and conse- 

 quently all they know about the development of their 

 picture is that "it took so many minims of No. i solution, 

 and so many drops of No. 2." 



Three chemicals are used principally in development, 

 viz., pyrogallic acid, a bromide either of potash or 

 ammonia, and an alkali. Pyro is the true developer. 

 Before it will do its work, however, it is necessary to 

 render it alkaline. The three alkalies commonly made 

 use of are (i) ammonia, (2) soda, and (3) potash. Potash 

 (in the form of a carbonate, of course,) is perhaps the 

 best to use, as it possesses the good qualities of the other 

 two without their faults. Ammonia is objectionable on 

 account of its volatility, while soda stains the gelatine 

 film yellow. The principal use ol the bromide is to 

 retard the action of the developer. 



In order, therefore, to mix the developer on scientific 



