VOL. LXXXV.] PHILOSOPHICAL TRANSACTIONS. 627 



triangles, preparations were made for taking the field; and the party which had 

 been engaged in the measurement of the base, were ordered to be attached to 

 the trigonometrical operation. Little difficulty was found in fixing on the choice 

 of the necessary apparatus. Lamps were constructed, by Mr. Howard of Old- 

 street, which were afterwards found to equal every thing which could be expected 

 from them. Instead of the reflector being exposed to the wind, these lamps 

 were inclosed in strong tin cases, having plates of ground glass in their fronts, 

 which effectually prevented the bad effects of an unequal and unsteady light. In 

 the centre of the back of each case, there were straps and semi-cylinders of tin, 

 which moving on joints, clasped the staff to which in their use they were braced. 

 Two of the lamps were of 1 1 inches diameter, and a 3d of 22 ; and the last of 

 these, prior to the use of it in the ensuing season, was lighted on Shooter's Hill, 

 and clearly distinguished at the distance of 30 miles. Copper nozles of different 

 sizes were also provided for holding the white lights. 



As it was easily foreseen that on eminences, on which it was certain the in- 

 strument would be placed, it would be hazardous to trust it in a receptacle of 

 slight construction, great pains had been taken to make the observatory strong. 

 It consisted of 2 parts, the interior one of which, or the observatory itself, was 

 8 feet in diameter, and its floor of a circular form, and from the sides of it 8 

 iron pillars rose to the height of 7 feet, which were connected at the extremities 

 by oaken braces. The roof was formed of 8 rafters, which united at the top, 

 having their ends fastened into the heads of the iron stauncheons, and were 

 otherwise sufficiently clamped. The sides and roof were each composed of 24 

 frames, covered with painted canvass, any of which could be removed at pleasure; 

 and the whole was covered with a tent formed of strong materials. 



Mr. Ramsden had considerably improved the great theodolite, which, in other 

 respects, is of the same dimensions and construction as that used by General Roy, 

 which has already been described in the Phil. Trans. The construction of the 

 microscopes render them very superior to those of that instrument; as the means 

 by which the image is proportioned to the required number of revolutions of the 

 micrometer-screw, and also the mode of adjusting the wires to that image, are 

 much facilitated. For the first, there are 3 prongs proceeding from the cell 

 which holds the object-glass; these, after passing through slits in the small tube 

 which constitutes the lower part of the microscope, are confined between 2 nuts 

 which turn on this small tube, so that by turning the nuts, the object-lens is 

 moved towards, or from, the divisions on the circle, as occasion requires. To 

 adjust the wires in the micrometer to the image; in the upper part of the body 

 of the microscope are 1 nuts, one sliding within the other. To the upper end 

 of the interior one the micrometer is fixed: and near the lower end are 3 prongs 

 similar to those above-mentioned, but something longer. These prongs pass 



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