OF PNEUMATIC APPARATUS. 185 



2167. Oourjou's I in proved Galvanometer, adapted for the lecture table. It con- 



sists of a firm mahogany base, furnished with levelling screws, on which is 

 placed a graduated metallic circle and coil of fine insulated wire ; in these 

 a pair of astatic needles, about 6 inches long, supported on an agate cap, 

 vibrate freely when connected with a battery. 3 5 to 5 



2168. BACHOFFNER'S GALVANOMETER, with astatic needles, on mahogany stand and 



glass shade, complete ...... 18 



2169. TORSION GALVANOMETER, the astatic needles of which are delicately suspended 



in a glass tube, with a torsion circle and key very delicately balanced, with 

 screw adjustment ...... 2 18 



2170. MELLONI'S MAGNETIC GALVANOMETER, improved by Prof. Wheatstone, with 



reading microscope for measuring very feeble currents of electricity 5 10 



2172. Galvanometer, for detecting and measuring electric currents by the extent of 

 deflection of a magnetic needle, when subjected to the action of a conducting 

 wire 076 and 10 



PNEUMATIC APPARATUS, 



FOR DEMONSTRATING THE PRINCIPLES OF ELASTIC FLUIDS, MORE 

 ESPECIALLY THE MECHANICAL PROPERTIES OF AIR. 



The following air pumps are of the newest forms, being nearly all on the double 

 piston principle of Professor Tate, now exclusively used where the highest amount of 

 exhaustion is required. 



By a recent improvement the valves are easily removed or replaced by the owner at 

 pleasure for purposes of cleansing, etc. 



The screws and attachments are all of the same thread, so that the several pieces 

 may be easily fitted to each other. 



When the pump is out of use for a length of time, a little oil should be applied by 

 pouring a small quantity, say half a tea-spoonful in the centre hole a (fig. 2193), p. 187, 

 when a few strokes of the piston will convey it over all the working parts of the pump ; 

 a little tallow should be rubbed over the edges of the receiver, before fixing it on the 

 plate ; stop-cocks should always be kept open, and when the pump is not in use the 

 various parts should be well cleaned, and the nut screws be screwed in at a and c to 

 prevent the admission of dust. 



The practice of testing pumps by means of the syphon gauge, though much in use, 

 is often deceptive, thus : a speck of air at the closed end of the gauge will give a 

 fallacious appearance to the action of the pump by depressing the mercury more or less 

 according to the size of the air-speck ; a full length gauge in which the mercury is 

 drawn up, though inconvenient on account of its length, is therefore far preferable where 

 a delicate test of vacuum is required. 



2173. Air Pump, small size, with receiver for preparing microscopic objects 12 6 



2174. AIR PUMP, single barrel, f-inch diameter, 5 inches high, 4^-inch. ground plate, 



mounted on mahogany stand . . . . . 110 



2175. EECEIVER, for the above . 030 



