ZS6 Account of the L[fi' and Lnhouri 



While Mr. Ramsden was employed on his dividing ma- 

 chine, he improved at the same time other instrm-nents. 

 The theodohte het'ore consisted merely of a telescope, turn- 

 inu; on a circle divided at every three minutes, by means ot 

 a vernier; but in the hands oi' Mr. Ramsden it has become 

 a new and perfect instrument, which serves for measuring 

 heights and distances as well as for taking angles. I saw 

 in his possession the largest and most wonderful of all theo- 

 dolites, employed by general Rov for measuring the triangles 

 which at present join those of France, and by which there 

 cannot be an error of a second, though it is only eighteen 

 inches radius. It is furnished with two telescopes, which 

 each turn on a horizontal axis, and by which the angles be- 

 tween objects more or less elevated are reduced to Itbe ho- 

 rizon, and measured. General Roy has lately measured the 

 angle between the pole star and the sides of his triangles, in 

 order to have the convergency of the meridians such as it is 

 in our oblate spheroid. These operations have already sho\\n 

 that the difterence between the meridians of the observatories 

 ef I'aris and Greenwich is {)' 20". 



The barometer destined for measuring the heights of 

 nioimtains has been much improved by Mr. Ramsden. His 

 nieth(.)d of marking at the bottom the line of the level, and 

 of looking at the top to the contact of the index with the 

 summit of the mercury, renders it possible to distinguish 

 the hvxndredth part of a line, and to measure heights within 

 2. foot. He showed JNl. de Luc that it is the summit of the 

 column, and not the part which touches the glass, that 

 oueht to be observed ; and he caused to be engraved a table, 

 which accompanies his barometers, and which, without cal- 

 culation, gives the heights of places according to the height 

 of the barcjmeter, and even for difierent deorces of heat. He 

 has simplified also, in the most ingenious manner, the ap- 

 paratus for carrying and supporting this portal)le barometer. 

 Various other philosophical machines liave been made by 

 Mr. Ramsden, and always with new improvements : such 

 as an electric machine ; a manometer for measuring the 

 density of the air ; an instrument for measuring inaccessible 

 distances, and which renders it unnecessary to measure a 

 base ; assaying balances which turn with a ten-thousandth 

 part of the weight ; levels exeeedingly sensible ; the optic 

 rectangle, prismatic eve-glasses where much fewer rays arc 

 lost than by the reflection of an inclined mirror, when it is 

 necessary to look on one side ; the dynamett,'", with which 

 he measures the magnifying power of a telescope, by apply- 

 ing before the eve-glass a small scale divided into hundredths 



of 



