344 Mr. Scrymgeour's Experiments to determine the Injiuence 



is admitted, these minute and delicate bodies are completely 

 drowned. All that is necessary for seeing these objects, even 

 in the ordinary compound microscopes (engiscopes), providing 

 they have sufficient magnifying power, is to employ a faint il- 

 lumination. If, however, the observer is desirous of examining 

 the structure and organization of them, of course he must use 

 an instrument of superior quality ; for in this case not only 

 sufficient magnifying power and proper illumination are ne- 

 cessary, but penetration and definition. 



LVI. Narrative of Experiments made soith the Seconds Pen- 

 dulum, principally in order to determine the hitherto un- 

 assigned Amount of the Influence of certain minute Forces on 

 its Rate of Motion, By Mr, James Scrymgeour. 



[Continued from p. 251.] 



"jVfY next inquiry was to ascertain whether a pendulum of 

 x '-"- the same length made any difference in its time, accord- 

 ingly as its weight was light or heavy. The only method of as- 

 certaining this experimentally, is to employ the principle of 

 convertibility adopted by Captain Kater in his experiments on 

 the pendulum. The convertible pendulum which I employed 

 was composed of two steel rods placed an inch separate from 

 each other, and firmly joined by cross pieces at the ends as 

 well as at intervals of their length. The knife-edges or por- 

 tions of cylinders were fixed between the rods, at the points 

 of oscillation and suspension. 



In the following experiments the knife-edges were used, and 

 the pendulum was adjusted, first with weights which, along 

 with it, made the whole weight 8 pounds 10 ounces. When 

 the heavy end of the pendulum was lowest, it vibrated exactly 

 with the clock pendulum at an extent varying from 3° to 2° # 5 ; 

 when the light end was lowest, at an extent of vibration vary- 

 ing from 2° to 1°'5. 



The pendulum was then transferred to the vessel in which 

 the exhaustion was to be effected. When exhausted to a mean 

 pressure of \\ inch of mercury, the pendulum, with the light 

 end lowest, vibrated with the clock pendulum at an extent of 



will be intensely illuminated though the quantity oflight is small; but if we 

 employ the flame of a large lamp, &c. at some distance from the object, its 

 intensity will be small, though the quantity of light be great. It will be 

 found generally preferable to employ a small quantity of intense light, ra- 

 ther than a larger portion of weak light; and, if possible, avoid the use of 

 lenses or mirrors, either for condensing or changing the direction of the 

 light. 



