Proceedings, xxix 



divided into two detachments, which in alternate order were 

 taken round first one and then the other department by members 

 of the respective staffs, to whose kindness and clear explanations 

 we were much indebted. 



On the astronomical side, the first object seen was the transit 

 instrument. This is a large telescope mounted between two 

 massive masonry pillars in such a way as to swing in a north 

 and south plane or meridian. The field of the telescope is 

 crossed by five equidistant vertical " wires " (really made of fine 

 spider's threads), and one horizontal one. The exact observation 

 of the position of a star requires the determination of the time 

 at which it crosses the meridian, and of its then distance from 

 the zenith. The time determination is made by the observer 

 pressing a knob each time that the star, as seen in the telescope, 

 crosses successively one of the five vertical wires ; this closes an 

 electric circuit, and makes a dot on a graduated paper fixed on a 

 revolving drum in another apartment ; the mean of the five 

 records gives a very accurate determination of the time when the 

 star crossed the meridian. The zenith distance or declination 

 is measured on an accurately divided platinum circle attached to 

 the transit instrument; this is observed through fixed micro- 

 scopes furnished with a comb-shaped micrometer arrangement, 

 by which the circle can be read to the fraction of a second of arc. 

 In the same room are preserved various astronomical instruments 

 of historical interest. Through a telescope in another room the 

 members obtained a view of the sun, on which a large sun-spot 

 was visible at the time. They also saw the dome in which the 

 large new refractor, with object-glass twenty-eight inches in 

 diameter, is to be mounted. 



In the meteorological department the most remarkable feature 

 is the series of self-recording instruments by which, through the 

 aid of photography, a continuous record of meteorological and 

 magnetic elements is obtained. In instruments in which the 

 amount of movement to be measured is very small, the movement 

 is made to alter the position of a small mirror, upon which a 

 beam of light falls, and is reflected to a sheet of sensitive paper 

 on a revolving drum ; the amount of movement as shown by the 

 spot of light being multiplied manifold by the angular displace- 

 ment of the mirror. The self-recording instruments and baro- 

 meter are placed in a cellar to avoid changes of temperature and 

 accidental vibrations. Before entering this, the party had to 

 leave behind their keys, knives, and other iron articles. Besides 

 the ordinary meteorological instruments, sunshine-recorder, 

 anemometers, &o., there are the earth thermometers, mentioned in 

 my paper on "Earth Temperatures" (Trans., Art. 110), the deepest 

 of which has its bulb at a depth of 25 ft. below the surface. 

 These do not require to be pulled up to take the readings, but 



