the governor now described. The governor at these works had given no trouble whatever, and had not required any attention; the speed of the engine was readily regulated to any number of revolutions required by simply altering the quantity of liquid in the chamber of the governor, and this was a great advantage for engines where the speed had to be varied frequently. Mr. L. Olrick considered the governor now described was both simple and effective, and could be applied where more complicated governors would not be suitable; and it appeared to possess several points of merit, that would be appreciated in practice, one of which was a very neat arrangement of throttle valve used in connexion with the governor. Although the governor now described appeared to him to be one of the best he had seen for all practical purposes, he considered the differential governor with one arm invented by Mr. Siemens was a nearer approach to perfection in maintaining the speed unaltered, on account of being dependent upon the time of a pendulum, which rendered it impossible for the engine to go beyond the prescribed number of revolutions per minute; and he was surprised that that governor was not found in all spinning mills, as it would effect so great a saving by preventing waste of steam and breakage of threads through variations of speed. As an illustration of the extreme sensitiveness of that governor he might mention that in one instance where it was in use the crank shaft broke while the engine was running at 70 revolutions per minute, and, though the whole of the work was thus suddenly thrown off, the number of revolutions only increased to 71 per minute. The President considered the governor described in the paper was remarkable chiefly for its simplicity, and it appeared to have been found to answer well in practice. Reference having been made to his own liquid governor in connexion with that now described, it was to be observed that, though both of them dealt with liquid resistance, they did so in a different manner. In the governor now described the to act power upon the throttle-valve was obtained in an indirect way; the rotating paddle-wheel did not act directly upon the valve, but impelled the oil against the corrugations in the casing containing it, and the impact tended to make the casing rotate in the same direction; the casing, however, was held back either by a dead weight, or, as had been suggested, by a spring, or really by a combination of a dead weight and a spring, because a weight alone would over-run itself if acting at a constant leverage. When therefore the velocity of the rotating paddle-wheel was so proportioned to the weight as just to hold the latter suspended, a balance was established; but as soon as the engine exceeded its normal speed, an additional amount of impact was created in the oil casing, which accumulated until it had sufficient power to overcome the resistance of the throttle valve and stuffing box on the valve spindle. This power, however, to move the valve was not large, in comparison with the total force acting to support the governor weight at the normal speed; if for instance 130 revolutions of the paddle-wheel per minute sufficed to balance a weight of 10 lbs., then a variation in speed of two or three revolutions per minute would affect the weight to the extent of only a small fraction of a pound, which would accordingly be the limit of the force available for moving the valve. It was therefore an object of primary importance that the frictional resistance in the valve and stuffing-box should be as much reduced as possible; and this appeared to have been accomplished successfully by the construction of the throttle-valve now described. If this delicacy of action could be maintained, the governor would be applicable no doubt to engines subjected to frequent and sudden alterations of load. His own liquid governor, consisted of a cup of parabolic section revolving upon a vertical spindle within a vessel partly filled with cil or water, and by rotation of the cup, which was open at top and bottom, the liquid was caused to rise up the sides of the cup, but did not overflow the edge until the speed of rotation had reached a certain limit. Up to the moment of the cup overflowing it acted only as a fly-wheel, but at the moment it overflowed it became a pump drawing in liquid through the central aperture at the bottom and discharging it over the top edge; the external surface of the cup and the interior of the vessel in which it revolved were provided with a series of radial vanes, and the overflowing stream of liquid from the cup impinged successively upon the stationary vanes and upon those on the revolving cup, thus presenting a practically uniform resistance to its rotation. The cup was driven by the engine through differential gearing, with which was also connected the weighted lever of the throttle-valve, this constant weight acting always to maintain the uniform rotation of the cup. Although a weight was thus employed both in his own and in the Allen governor, there was an essential difference of action between the two, inasmuch as in the Allen governor the throttlevalve had to be moved by only a fractional portion of the suspended weight; whereas in his own governor the difference between the uniform rotation of the cup and the varying speed of the engine acted direct upon the valve through the differential gearing, the uniformly rotating cup serving as a fulcrum or abutment, while the actual amount of the weight upon the throttle-valve lever was immaterial, except as regarded the original determination of the frictional resistance of the cup. It would thus be seen that there was indeed more similarity in appearance between the two governors than really existed in their modes of action; and it was clear that the throttle-valve now described in connexion with the Allen governor must be looked upon as an essential part of the governor, the prompt action of the governor depending upon the ease with which the valve could be moved with a slight amount of force. Owing to the great simplicity of this governor, and the careful manner in which the mechanical details had been worked out, he had no doubt that it would meet with success in its application. The Allen Governor is now in use in many iron works, cotton mills, paper mills, and other works, in the United Kingdom and in foreign countries. OBSERVATIONS ON THE ELVAN COURSES, GREEN- Member of the Miners' Association.* In the following paper an attempt has been made to describe the mineralogical and physical characters of the Elvan Courses of that part of Cornwall contained within a line drawn from Watergate Bay to Par on the east, and another drawn from St. Ives to Penzance on the west (including the Elvans cut through by these imaginary lines), noticing the metalliferous deposits in connexion therewith, and the effect produced by these rocks on the lodes in their vicinity. The Elvan Courses, from their occurrence in connexion with the principal tin and copper mines of the county, and their intimate association with the lodes in the district they traverse, have long been objects of especial study and interest, their structure and mineral composition rendering them very conspicuous; and from their occurring in dykes frequently underlying at a considerable angle opposite to the dip, but coincident with one set of divisional joints of the slate which gives them well-defined walls, their appearance is still more remarkable. The Cornish Elvans are rocks occurring in veins or dykes composed of two or more of the following minerals in variable * For this paper and its accompanying maps and large collection of rocks an award of the Society's first-class Silver Medal was made to Mr. Barnett. proportions :-quartz, felspar, mica, schorl, and chlorite; the base being granular or compact, and generally containing imbedded crystals of its components, which renders it porphyritic. The elvans may be regarded as consisting essentially of the same chemical composition as granite, differing-as in that rock —in the variety and proportions of their constituents, the same elvans frequently undergoing considerable changes of structure and composition according to the character of the rock cut through; those in granite being generally finer grained and more closely allied to that rock, than those in the slate. The base of the light-colored elvans is felspathic or quartzo-felspathic, but chlorite and schorl sometimes enter into the composition of the darker varieties. De la Beche considered them to be mere granitic dykes, the chemical composition of which bears great analogy to that of the chief granitic masses in the vicinity of which they occur, their mineral structure considerably depending upon the conditions for cooling to which they have been exposed, their central portions being, therefore, often more generally crystalline than their sides. In nearly all elvans, especially those containing a large per centage of quartz, the outsides of the dykes are harder, more compact, and do not decompose readily on exposure; probably owing to the different molecular arrangements of the constituents, the walls of the dykes having been suddenly cooled by radiation of their heat, the particles are more intimately associated, forming a quartzo-felspathic rock; whilst, in the interior of the mass, the quartz particles aggregate together or crystallize, leaving a felspathic base which decomposes more easily; but in elvans composed chiefly of felspar, the outsides of the dykes decompose quite as rapidly as the central or more crystalline portions. The elvans are of very variable degrees of hardness, principally depending, it would appear, on the condition of the felspar. [* Geological Survey of Cornwall, Devon, and West Somerset, page 184. |