12 EIGHTH UEPORT — 1838. 



at considerable length, every particular connected with his kyel- 

 ino- from Sheerness to London ; a scientific enterprise of similar 

 character to that in which I was about to engage. This memoir 

 of Captain Llo5'd I regard as one of considerable value, and 

 have derived from it much information and assistance. Most 

 of his arrangements appear to me to be very judicious, and se- 

 veral of them I have either adopted or imitated. On one im- 

 portant point, however, I am obliged to differ from him, to 

 which I shall have occasion to advert presently. 



The instruments made for this undertaking were a spirit-level, 

 and brass leveling- staff, by Simms, London. The telescope, 

 though only 14 inches in length, was found to bear the high 

 nia"'nifying* power of 26 so well under all circumstances, that 

 the other eye-piece with which it is furnished was never em- 

 ployed. The glass spirit-tube is so nicely ground, that the 

 position*of the air-bubble is sensibly altered by raising or lower- 

 ing either end of the tube jo^th part of an inch. In the focus 

 of the telescope are a horizontal and two vertical hairs, which 

 latter afford a very convenient means of measuring the distance 

 of a station, within about the y^pth part of the truth, by count- 

 ino- the number of intercepted divisions of a scale made for the 

 purpose, and held horizontally over the station by an assistant. 



The legs which were made to support the level, although very 

 strong, were found to vibrate so much from the action of the 

 wind, as to render it difficult to take a correct observation, ex- 

 cept in perfectly calm weather. It was also next to impossible 

 to level the spirit-tube, unless by accident, for want of a slower 

 and more delicate motion than that afforded by the parallel 

 plate screws. I therefore ordered a very strong stool to be made 

 by a carpenter, the top of which was a thick board 12 inches 

 in diameter. The level was then detached from its former sup- 

 port, and fastened to a circular piece of mahogany, which rested 

 by three foot-screws on the top of the stool, and was firmly se- 

 cured to it by a stout wooden screw, with a nut at bottom, 

 passing through both the circular boards. On trying this appa- 

 ratus, I found that a more delicate vertical motion was still 

 wanted, which was at length perfectly attained by causing one 

 of the three foot-screws to rest on a small brass lever at a very 

 short distance from the fulcrum, while tiie farther end, furnished 

 with a fine screw and milled head, communicated about j^ th of its 

 own vertical motion to the foot-screw of the level, affording a 

 very simple and delicate means of adjustment. 



The level, although now incomparably steadier than before, 

 was still found liable to disturbance from the wind, when it blew 

 with any considerable force ; to protect it from Avhich we car- 



