148 SOME REFINEMENTS OF MECHANICAL SCIENCE. 



condition possible. Instead of the large circles and sectors used by 

 the ancients, circles of smaller diameter have been made as the 

 methods for graduating have been improved, until those of the more 

 modern instruments are seldom greater than 80 inches, and some of 

 the latest meridian instruments have circles of but 25 inches. 



The smaller circles, which can be made and graduated with greater 

 precision than the larger ones, are also less liable to change in form, 

 owing to their weight and the variation in temperature, and with the 

 aid of the reading microscope the results obtained would not be 

 possible with the larger circles. 



A 25-inch circle read with a microscope having a power of 40 

 would be equivalent to a circle of about 80 feet in diameter, and 

 a single second of arc as seen through the microscope Avould be equal 

 to 0.0024 of an inch, a quantity easily subdivided. 



xV most important adjunct to the astronomer's instrumental equip- 

 ment is the filar micrometer. With it he determines the errors of 

 divisions, the eccentricity of his circle, and measures the angles to 

 within a fraction of a second ; and when used at the eye end of the 

 telescope he determines the positions and motions of the stars and 

 the distances and diameters of the planets. In these little instru- 

 ments, whether of the simple or complex form, the chief requisites 

 are the screw and the ci'oss wires, for upon them the value of the 

 observations and measurements depend. 



To make the screw of a micrometer so true that the errors in the 

 threads can not be detected by its own magnifying power is an 

 extremely difficult task. These micrometer screws are often made 

 with 100 threads to the inch, and are provided with graduated drums 

 having 100 divisions, the readings being made in tenths of a division. 



The cross w^ires, which are but common spider lines, because of 

 their fineness and the remarkable qualities they possess, are indispens- 

 able in micrometric work. 



That the repulsive and even dangerous spider has plenty of ene- 

 mies among the human familj^ there can be no doubt, yet if the value 

 of the contributions which it has made to the cause of science was 

 generally known, it would surely have a greater number of friends 

 than at present, and most certainly the astronomer will say naught 

 against it, for after the experience of many years he has found that 

 the spider furnishes the only thread which can be successfully used 

 in carrying on his work. 



The spider lines mostly used are from one-fifth to one-seventh of a 

 thousandth of an inch in diameter, and in addition to their strength 

 and elasticity, they have the peculiar property of withstanding 

 great changes of temperature, and often when measuring the sun 

 spots, although the heat is so intense as to crack the lenses of the 

 micrometer etepiece, yet the spider lines are not in the least injured. 



