40 



ON THE CONSTRUCTION AND USE OF 



'A 





ments in the rod e, to produce equal changes in the flow. The rating requires 

 consequently only a few moments. 



The object of the side tube /is to avoid disturbing d when it becomes necessary 

 to refill the cylinder, for when it is once opened to the right degree, it hardly 

 requires to be touched again during a night's work. In order to arrest the down- 

 ward motion of the piston at any point, a clamp screws on the piston rod, and can 

 be brought into contact with the cylinder head, as in the figure. 



That this instrument should operate in the best' manner, it is essential to have 

 the interior of the brass cylinder polished from end to end, and of uniform diameter. 

 If any irregularity should be perceived in the rate of going, it can be cured com- 

 pletely by taking out the piston, impregnating its leather stuffing with fine rotten 

 stone and oil, and then rubbing it up and down for five minutes in the cylinder, so 

 as to restore the polish. The piston and cylinder must of course be wiped, and 

 regreased with a mixture of beeswax and olive oil (equal parts) after such an 

 operation. In replacing the piston, the cylinder must be first filled with water, to 

 avoid the presence, of air, which would act as a spring. 



Although it may be objected that this contrivance seems to be very troublesome 

 to use, yet that is not the case in practice. Even if it were, it so far surpasses any 

 prime mover that I have seen, where the utmost accuracy is needed, that it would 

 be well worth employing. 



c. T7ie Sun Camera. 



In taking photographs of the sun with the full aperture of this telescope, no 

 driving mechanism is necessary. On the contrary, the difficulty is rather to arrange 

 the apparatus so that an exposure short enough may be given to the sensitive plate, 

 and solarization of the picture avoided. It is not desirable to reduce the aperture, 

 for then the separating power is lessened. The time required to obtain a negative 

 is a very small fraction of a second, for the wavy appearance produced by atmos- 

 pheric disturbance is not unfrequently observed sharply defined in the photograph, 

 though these aerial motions are so rapid that they can scarcely be counted. Some 

 kind of shutter that can admit and cut off the solar image with great quickness is 

 therefore necessary. 



In front of an ordinary camera a, Fig. 36, attached to the eyepiece holder of the 

 telescope, and from which the lenses have been removed, a spring shutter is fixed. 

 It consists of a quadrant of thin wood b, fastened by its right 

 angle to one corner of the camera. Over the hole in this 

 quadrant a plate of tin d can be adjusted, and held in position 

 by a screw moving in a slot so as to reduce the hole if desired 

 to a mere slit. It may vary from 1| inch to less than -^ of 

 an inch. The quadrant is drawn downwards by an India- 

 rubber spring g, 1 inch wide, J of an inch thick, and 8 inches 

 long. This spring is stretched when in action to about 12 

 The Spring Shutter. inches, and when released draws the slit past the aperture c in 

 the camera. Two nicks in the edge of the quadrant serve with the assistance of a 

 pin e, which can easily be drawn out by a levqr (not shown in the cut), to confine 



Fig. 36. 



