436 SCIENTIFIC EXPERIMENTAL APPARATUS. 



ends, and the weiglit of tube and inclosed rod is taken oif tlie Y's of the 

 apparatus by straps and spring balances. 



The measuring part of the apparatus is entirely distinct^ and at a dis- 

 tance from the tube. The tube is only placed in contact with the measur- 

 ing api)aratus at the moment of making the measurement. 



The measures are made by Saxton's reflecting comparator, or by mi- 

 crometer screws brought up to the ends of the rod, and contact deter- 

 mined by the closing of a voltaic circuit. 



Successive contacts, made by successively placing the rod (cooled to 

 melting ice), give a range of measures on a rod one metre long, equal to 

 about l-30,000th of an inch. Micrometer screws, pitch l-50th and 1-lOOth 

 inch. 



2. Vertical comparator, with contact lever, micrometer screw, pitch 

 = 1-lOOth inch. Well made, heavy cast-iron frame of T section, mounted 

 on a cast-iron tripod base. 



3. Apparatus for the comparison of yard and metre (end or line meas- 

 ures) by Airy's method. Massive cast-iron plate with guides, two mi- 

 crometer microscopes, with apparatus for handling measures. 



4. Spectrometer. — Circle divided by Brunner of Paris, reading by ver- 

 nier to 5". Telescopes: one set, one inch aperture, 10-inch focus. 

 Another set : collimator, 5 feet long, and observing telescope, 18 inches 

 focus. {Property of Professor Mayer.) 



5. Interferential refractometer of Arago. — Tube of one metre for holding 

 gas whose index of refraction is to be measured. Collimator, six inches j 

 obser^'ing telescope, three-quarters of a metre focus. 



6. Two of Sipp^s chronoscoyes. 



7. Electric clocJc, to be used in connection with inductorium and rotat- 

 ing cylinder, to determine the number of vibrations of tuning-forks and 

 other solid bodies. 



8. Spherometer, by Brunner, of Paris, furnished with a contact lever. 

 Pitch of screw 0.5 millimeter. 



9. Full sets of resistance coils, condensers, and high and low resistance 

 Thomson galvanometers. 



Massachusetts Institute of Technology. — Department of 

 Physics, Boston, Mass. 



(In charge of Professor Charles R. Cross.) 



1. Cathetometer, by Staudinger. Scale graduated to millimetres ; 

 length, 1,020 millimetres. Yernier reads directly to l-20th millimetre. 

 Focal length of telescope is 280 millimetres. One division of level cor- 

 responds to 5". 



2. Dividing engine, by Buff and Berger. The microscope is carried by 

 two screws moving at right angles to each other, so as to give two co- 

 ordinates. The pitch is l-20th inch, and the length of the screws, 16 

 inches and 4.5 inches respectively. Circles are di\'ided into 100 parts. 



