270 Dr. Draper ow the Use of a Secondaiy Wire as a 



same vertical straight line, through whatever arc the micro- 

 meter may have moved. Now it would be very difficult to 

 accomplish this by any system of adjustments. 



Whether the instrument is arranged with one or several 

 needles, or whether it has a coil or merely a single strap, the 

 vertical distance from the coil or strap, when the index is 

 brought to zero, ought under no circumstances to vary. 



In a climate as hot as that in which the following experi- 

 ments were made, one of the most unpleasant deviations de- 

 pends on the thread wrenching in the wax, which is used to 

 fasten it to the needles at one end, and to the micrometer at 

 the other; when the wax softens, and the thread is moved 

 through several degrees, it is not the free part alone that un- 

 dergoes torsion, but also that which is in the wax, hence 

 arises an error as respects the zero point. This I have al- 

 ways avoided, by ascertaining the zero at the beginning and 

 close of each experiment. 



After having had some experience with voltameters, de- 

 flecting galvanometers, &c. I am induced to describe the in- 

 strument used in these experiments, for it will enable those 

 who are not accustomed to the torsion balance to execute 

 measures very easily, which they might otherwise ineffectually 

 attempt. 



A A, BB(Fig. I.Plate I.) is a glass jar, ] 6 inches high, open at 

 both ends; at A A it is 2|^ inches in diameter, at BB, 6 inches ; 

 it rests upon a piece of wood 8 inches by 10. A strap of stout 

 sheet copper, effe, 1 inch wide and 15 long, is bent into the 

 form indicated ; its extremities at ^ e being let into the wood, 

 and bearing mercury boxes, D D. The central part of this 

 strap from fX.of is placed horizontally, and has a circular 

 apei'ture and side gap, as is shown in fig. 2. a a, through 

 which the spindle carrying the needle can be passed, and 

 works. 



The upper extremity of the jar A A, is accommodated with 

 a divided circle, in the centre of which the key G works : this 

 key is ground like a stopcock to a slightly conical figure, it 

 therefore revolves very truly without any shake ; it is drilled 

 longitudinally to admit the passage of the glass thread, which 

 is secured in it by means of a perforated straw and a drop of 

 sealing-wax. 



The other extremity of the thread enters a litde tubular 

 perforation in the ivory axis w w', and is also secured therein 

 by wax. Only one needle is used, it is lozcDge-shaped and is 

 4>i inches long. Besides carrying this needle, the ivory axis 

 extends an inch and a half below it, and in a slit at its lower 

 extremity, confines a parallelogram of stout tinfoil, r r, an 



