156 Account of Professor Carlini's 



may be measured without touching it ; -without approaching 

 it; without even opening the case \vhich contains it. > The 

 measure is obtained by bringing the wires in contact with the 

 images of the knife-edge suspension, and of the upper and 

 lower borders alternately of the platinum disk suspended to 

 the thread: thus preventing the risk of deranging the equi- 

 librium, and avoiding the effect which the heat of the body 

 might have on the very dilatable metallic threads ;ri J is 93n'>2C1i 



2. The half sum of the distances taken betweeri the stts- 

 pension, and the upper and lower edges of the disk, gives the 

 distance of the centre of the disk itself, without measuring its 

 diameter with a compass, an operation exceedingly difficult to 

 execute with the necessary precision. By this apparatus of 

 microscopes the length may be measured at pleasure, even 

 during the time of oscillation ; and being attached to the wall, 

 instead of supported by the floor, the risk of derangement by 

 the tread of the observer is avoided. : n.nn )f->! 



3. The pendulum, and the clock by which its oscilktions 

 are measured, were not, as usually, near together and resting 

 on the same base, but were perfectly separated. The coinci- 

 dences of the oscillations were observed, by bringing the image 

 of the pendulum of the clock, reflected by means of an oblique 

 mirror, in contact with the image of the simple pendulum seen 

 direct through a telescope. By this modification the risk of 

 the mutual influence of the pendulum and the clock is avoided. 



4. The disk was attached to the thread by means of knots 

 in the thread itself; avoiding the correction for the small cup 

 usually employed for that purpose. 



5. An alteration was made in the weight and shape of the 

 knife-edge suspension ; reducing its weight to about 10 grains, 

 and giving it the shape of a rotella, instead of that of a triangu- 

 lar prism. 



The simple pendulum and microscopes were attached to a 

 strong wall, in a room on the ground floor, contiguous to the 

 temporary observatory, and well sheltered from the sun and wea- 

 ther. The clock with which the pendulum was compared, was 

 supported by a pyramid of masonry resting on the ground, and 

 occupying the middle of the room. The experimental length 

 between the microscopes was referred to three standard metres^ 



