MADE AT HUDSON OBSERVATORY. 46 



upon the pier, and is secured immoveably to it by stout screws entering brass 

 sockets, which are leaded to the stone. In the focus of the telescope are five 

 vertical equidistant spider lines, besides the micrometer, and they are crossed 

 by five horizontal ones. There are three eye-pieces, one of them being a diago- 

 nal eye-piece, which I almost exclusively employ; and they may be slid back 

 and forth so as to be brought opposite either of the vertical wires. The object 

 end has a cap pierced with two apertures. The level for securing the hori- 

 zontality of the axis is a rider of seventeen inches length, and is accompanied 

 by a small bubble at right angles. My observations make the value of the 

 division of the level 1".278. I have two meridian marks, one to the north and 

 the other to the south; the former distant about sixty rods, and the latter 

 nearly a mile. The circle is eighteen inches diameter, with six radii, and is 

 firmly connected with another of equal size, but not graduated, separated by 

 an interval of three and a half inches, and between the two is the telescope. 

 The graduation of the circle is on platina to five minutes; and there are three 

 reading microscopes, each measuring single seconds. These microscopes are 

 of the kind called Troughton's reading microscope, and are represented in 

 Pearson's Practical Astronomy, Plate XI., Fig. 9. They are screwed upon a 

 stout brass circle attached to the frame, and may be set to any part of the limb. 

 Microscope A, which carries the pointer, I have set to indicate the polar point; 

 microscope B at 120° north polar distance; and C at 24"0°. To the frame which 

 sustains the microscopes is permanently attached a delicate spirit level, point- 

 ing north and south. 



The equatorial telescope, made also by Simms, is five and a half feet focal 

 length, with an object glass of 3.8 inches clear aperture. It has six celestial 

 eye-pieces, with magnifying powers from 20 to 400 ; a terrestrial eye-piece ; an 

 eye-piece with five parallel spider lines, crossed by as many others at right 

 angles; and a position micrometer, represented in Pearson, Plate XL, Figs. 1, 

 2, 3. A lamp, suspended from the side of the tube, illumines the field of view, 

 when it is necessary to use the micrometer by night. The frame of the equa- 

 torial is of cast iron, secured immoveably to the pin by screws entering sockets 

 leaded into the stone, It was made for the latitude of the observatory, and the 

 polar axis admits but a slight motion in altitude and azimuth, by means of 

 screws at its lower extremity. The right ascension circle is twelve inches 

 diameter graduated to single minutes, and reads by two verniers to single 



VII. — M 



