ON GRAPHIC METHODS IN MECHANICAL SCIENCE. 415 



placed in a sort of cage, so as to protect it, but at the same time to allow 

 the atmosphere to act upon it, this cage being attached to the box holding 

 the self-recording portion, so that it can be placed in bath-houses, 

 breweries, &c. An instrument acting on a similar principle, which 

 allows a communication with any vessel in which it is desired to register 

 the temperature of the liquid, is also largely used. Still another variety 

 of the same class of instrument acting by means of the expansion of the 

 liquid, is arranged for obtaining the temperature of drying and other 

 enclosed chambers, a rod containing the liquid being inserted in the 

 enclosed space, a self-recording portion being placed outside. Probably 

 one of the most beautiful instruments of this kind is the recording 

 pyrometer designed by M. Saintignon, and described in the report of 

 Colonel Sebert already alluded to. This pyrometer acts upon the differ- 

 ential principle, a reservoir of the water at constant height being placed 

 about 9 or 10 feet above the place of which the temperature is required. 

 The water passes first through a filter divided into two parts, the ex- 

 ternal compartment being that in which the liquid circulates, and which 

 acts as an envelope to cool the water after it has passed through a hollow 

 tube placed in the oven or furnace the temperature of which is required 

 to be known. It then passes out, acting on the bulbs of two air ther- 

 mometers, one of which communicates with the outer partition of the 

 vessel above mentioned. By means of a delicate aneroid there is thus 

 obtained a graphic record of the temperature of the furnace. 



A hygrometer is also described, composed of two thermometric tubes 

 placed one beside the other ; one rests in the dry, and the other is main- 

 tained at constant dampness by means of a cotton wick descending into a 

 metal basin by the side of the instrument. The thermometric tubes each 

 have the self-recording pencil marking the same drum revolving by 

 clockwork ; the difference between the two records is the measure of the 

 humidity of the air. To this instrument the name of ' Psychrometer ' 

 has been given. 



A self-recording marine thermometer has been used by M. Pouchet, 

 director of a French marine laboratory, which can be submerged to a 

 depth of 120 feet, being raised every fifteen days for changing the paper 

 of the recording portion. This is also described by Colonel Sebert, being 

 made and designed by Messrs. Richard Freres. 



Coming under this head may be classed the various types of sunshine 

 recorders, the simplest of which was designed years ago by Sir G. G-. 

 Stokes, Bart., consisting of a sphere which focussed the sun's rays upon 

 a graduated strip of paper, so as to give the intensity of the sunshine at 

 various times. Other and improved instruments of the self-recording 

 type have been since designed for this purpose. 



/. In recent years a careful study has been made, by means of self- 

 recording instruments, both of the shocks resulting from earth- tremors or 

 earthquakes, and of the vibrations in locomotives, vessels, and machinery. 

 This work has been done through the agency of various self-recording 

 instruments, which have cleared away a great number of erroneous ideas 

 regarding earthquake shocks and vibrations generally. 



Self-recording instruments for examining earth-tremors, or seismo- 

 graphs, have principally originated with the professors who have been 

 fortunate enough to find a field for their labours in Japan," where these 

 phenomena occur so frequently as to give tolerably constant employment 

 to self-recording instruments. Professor Ewing and Professor Milne 



