ON THE KEW OBSERVATORY. 345 



would create a tendency to a vacuum in e^, which would cause air to flow 

 through e', &c. into e^ ; and that when all the water had descended into e^, 

 air would cease to flow into e^ 



e" is a pipe entering e*, and opening into the now lower part of e^, but 

 having at present no communication with e^ (for there is no opening in the 

 lower side of e^ at this point). 



e^^, a pipe entering e'", and opening into the now lower part of e^ but at 

 present having no communication with e^ (for there is no upper opening in 

 e^ at this point). 



It is therefore also evident, that if the vessel e® were made to take the 

 exact position of e^, by simply reversing the whole aspirator E, i. e. by 

 turning it on the pieces e* and e'", the pipes e" and e° (which open into e^) 

 would occupy the positions now occupied by e* and e'^, and that conse- 

 quently air would again flow through e', &c. 



But e' communicates with D, D, with the cistern A (fig. 1), and A with 

 C, therefore air would flow through C, would rise (in bubbles) through the 

 aether in A, carrying with it a large quantity of aethereal vapour, and would 

 pass through a*, D, and e', &c. (fig. 3) into e^, and so on. 



F (fig. 2) is a strong table in two perpendicular legs of which notches are 

 cut for the reception of the ends of e^. 



yy are plates, fitted to squares at their ends and screwed upon those legs, 

 in order to prevent e^ from rotation when e^ and e^ are made to exchange 

 positions. 



It is not necessary to enter upon further details here. The construction 

 presents no difficulties (the kind of work required being principally that 

 which is commonly executed by makers of stop-cocks, valves, &c); the parts 

 of e* whereon e* and e'o revolve are slightly conical, and clamping nuts are 

 employed. 



The apparatus is very effective and convenient. With aether of second- 

 rate quality the mercury has been made to descend from about 63° to about 

 13°. Even pyroligneous tether or good naphtha are efficient, but act in- 

 juriously upon the metal. 



The lower part of A remains bright wlien the other part begins to be 

 clouded with dew, thus affording the advantage of contrast in the discovery 

 of the dew-point, but the line of demarcation is not always perfect at first. 

 Further improvements relative to this object are contemplated. The freezing- 

 point of B (as determined on April S, 1851) is at 32°'2. 



/in Hygrometric Sliding Rule, invented by Mr. Welsh (of this Establish- 

 ment), has been added to our stock of working instruments. The following 

 is his abstract from his description of this ingenious contrivance. 



" The hygrometrical sliding-rule has been devised for the purpose of facili- 

 tating the calculation of the results of observations with the moist-bulb hy- 

 grometer. The instrument is founded on Dr. Apjohn's formula, Dalton's 

 values of the elasticity of vapour being adopted. It has been so arranged 

 as to give by inspection the values of the following expressions : — 1st. The 

 elastic force of the aqueous vapour. 2nd. The temperature of the dew- 

 point. 3rd. The relative humidity of the air or the ratio to complete satu- 

 ration ; and 4th. The weight of water contained in a cubic foot of air. The 

 scales were divided on metal at Kew by the aid of Perreaux's dividing- 

 engine, and afterwards copied upon wood by the optician." 



A Standard Thermometer by M. Regnault, No. 231, has been received 



