48 REPORT — 1873. 



coated with dull black, which thermometer is enclosed in a glass jacket, the bulb 

 being in the centre of a sphere of not less than two inches diameter, and fi-om 

 which jacket nearly all the air has been exhausted. To all thermometers thus 

 mounted the title of vacuum thermometer has been applied. It has, however, been 

 found that the amount of exhaustion varies considerably, and that the indications 

 of the thermometer are thereby greatly affected. Yet the instruments hitherto 

 made have been indiscriminately sold and used, and no ready means have been 

 available for determining the amount of air left in. 



The speciality of the instrument now exhibited is, that a small vacuum-gauge is 

 inserted in the jacket, so that the precise extent to which the exhaustion has been 

 carried can be seen at any time, and strict comparability in this important respect 

 ensured. 



On a Oomjyound-Pendulum Apparatus. By S. C. Tisley. 



This apparatus was originally designed for the purpose of recording the figures 

 shown in Lissajous's experiments vdth. tuning-forks. 



The method of obtaining the vibrations is by means of two pendulums, which 

 work upon knife-edges, the supports being secured to two sides of a piece of maho- 

 gany, so that the pendulums swing at right angles to eiich other. The pendulums 

 are about 3 feet long, and are continued above their supports about 8 inches, 

 finishing at their tops in ball-and-socket joints. \\'we arms are screwed into the 

 ball-and-sockets, and connected with a pen or tracer. When at rest, the two pen- 

 dulums and tracer are at three corners of a square. One pendulum has two sliding 

 pans for holding weights, one above the point of suspension and one below ; the 

 other pendulum has two sliding pans, but both below the point of suspension ; four 

 weights are generally used, each weighing about "2^ lbs. 



When a single weight is placed on eacli of the bottom pans and properlj^ ad- 

 justed, the vibrations of the two pendulums being equal, the figure formed by the 

 tracer wiU be an elliptical spiral, gradually dying out so as to produce a watch- 

 spring-shaped curve. A small sliding weight is attached to the first pendulum ; 

 and by moving this up or down, the viljrations can be brought perfectly into unison, 

 or thrown slightly out of time, thus producing through the tracer a variety of com- 

 plicated and interesting figures. The second pan is used for varying the rates of 

 vibration of the two pendulums in certain ratios, so as to produce curves of diflerent 

 characters. A variety of tracings illustrating this were exhibited. 



The use of the pan above the point of suspension is of great value, as it gives a 

 ready means of altering the proportions. Thus by moving the weight {21 lbs.) from 

 a pan below to one above the point of suspension, and placing a balance-weight of 

 A lb. on the lower pan, the pendidums having originally been adjusted for unison, 

 the resulting vibrations will be in the ratios of 3 to 1 ; and if they had been ad- 

 justed to 3 to 2, the result would be 2 to 1, and so on. 



In the table under the tracer a glass plate is let in, so that, by placing a reflector 

 below and above, a light can be thrown through the object, and a magnified image 

 produced on the screen during its formation; in that case blackened glass and a 

 needle-point for tracer are used. 



On a new form of Pendulum for e.vliihiting Superposed Vibrations. 

 Bij Professor A. S. Heeschel, B.A., F.R.A.S. 



The contrivance exhibited originally presented itself to the author at the Obser- 

 vatory of R. S. Newall, Esq., Gateshead-upon-Tyne, where the observing-chair is 

 supported bv a counterpoise consisting of a horizontal iron bar loaded with weights, 

 and fastened at its two ends to wire ropes, which, passing over two pulleys, support 

 the chair. When the chair-frame was moved, the two ends of this pendulum 

 showed themselves to be capable of three modes of vibration — one longitudinal (in 

 the direction of the bar's length), and two transversal ones proceeding from the 

 bar's displacement either angularly about its middle point or parallel to itself. 

 The combination of the first two of these movements together made the end of the 



