n6 POPULAB SCIENCE MONTHLY. 



designs of and by processes invented by Prof. Rowland, who was 

 constantly at hand to direct every movement. This machine is in 

 a dark vault under the laboratory. When a " grating " is being 

 made, it runs night and day. The vault is locked, and no one is 

 allowed to enter it, for the machine is so sensitive that the tem- 

 perature of a human body would disarrange it. When a new 

 diamond point is being tested, as is now the case, Prof. Rowland 

 will permit a few people to visit it. Sir William Thomson, the 

 Earl of Rosse, Lord Rayleigh, Prof. Ball, Astronomer Royal of 

 Ireland, the late Prof. Helmholtz, of Berlin, Prof. Mascart, of 

 Paris, and Prof. Lemstrom, of Sweden, are among those to whom 

 this courtesy has been extended. The motive power of the ma- 

 chine is a hydraulic engine. The water is kept at a constant 

 height in a tank near the roof, to insure unvarying speed. It is 

 driven by a belt attached to a solid brass driving wheel on the 

 machine. A crank is turned by the same on the other end of the 

 shaft. This crank moves the carriage that conveys the diamond 

 point back and forth over the surface of the " grating " or plate. 

 This carriage rests on two steel ways, which are flat on top and 

 slanting slightly outward, so that there are three points on one 

 way or rail on which the carriage rests. These "ways" are 

 ground so as to make them as nearly accurate as possible. But 

 they can not be made perfect, for Mr. Rowland tested them with a 

 microscope and found that they were " out " that is, not exactly 

 perfect by one fifty-thousandth of an inch. He did not attempt 

 to improve them. 



One of the most difficult problems that Prof. Rowland and Mr. 

 Schneider have to solve is to find a diamond point that is exactly 

 right. Some are too blunt, some have one defect, some another, 

 and it generally takes from two to eight months to find an avail- 

 able diamond. 



As the diamond carriage moves exactly in the same line back- 

 ward and forward every time, the metal plate or grating beneath 

 must move slightly when the diamond makes a stroke. These 

 tiny grooves must be exactly the same distance apart, and as 

 there must be from ten thousand to forty-eight thousand parallel 

 grooves or lines made within the space of one inch, it is readily 

 seen that the lateral movement of the metal plate is very small. 

 At every stroke of the diamond, the carriage carrying the plate is 

 moved by means of a steel screw. It is the only absolutely exact 

 screw ever made. The " ways " mentioned above, when tested by 

 the microscope, are one fifty thousandth part of an inch w out * of 

 the exact, but the strongest microscope can find no flaw in the 

 exactness of the screw. In order to manufacture this screw, it 

 was necessary to make it under water, which was kept at a cer- 

 tain temperature. If it had been made in the air, or the tempera- 



