154 



HOROLOGY. 



i.'ompenia- by cold, the mercury will also be contracted by it ; and 

 tion hence the lengthening or shortening of the pendulum 

 | ieat of co ^ ; s compensated by equal and oppo- 

 site expansions or contractions in the mercury. 



cm u urns. 



Gridiron 

 pendulum. 

 PLATE 

 CCCVII. 



V'S, 



Dimensions 

 of the grid- 

 iron pendu- 

 lum. 



Gridiron Pendulum. 



The gridiron pendulum invented by Harrison, is 

 composed of nine round rods, five of which are made of 

 steel, and four of brass ; and is represented in Fig. 2. 

 where the steel rods are distinguished from the brass 

 ones by a darker shade. As it somewhat resembles the 

 common gridiron in appearance, it hence probably recei- 

 ved its present name. Not many years after this was pro- 

 duced, the French artists contrived a variety of com- 

 pensation pendulums, but the gridiron seems to be the 

 one now general ly adopted by them. The first pendul um 

 of this kind which we made was nearly thirty years ago; 

 and knowing that Mr Camming had some practice in this 

 way, he was applied to, and very obligingly sent a draw- 

 ing, and the different lengths for the brass and steel 

 rods, which, on being tried some years afterwards, by 

 means of a transit instrument, was found, on the whole, 

 to be tolerably correct, but yet not quite so accurate in 

 the compensation as could have been wished. From 

 an abstract of the going of a clock with this pendulum, 

 it appeared, that, during the temperature from 46 to 48 

 degrees of Fahrenheit's thermometer, it kept mean 

 time A temperature 10 degrees lower, made it gain at 

 the rate of nearly half a second in a day, and 10 de- 

 grees higher made it lose about as much. 



The lengths of the rods were, outside steel rods from 

 pin to pin 29.5 inches ; centre steel rod from upper end 

 of the pendulum spring to the pin at the lower end, 31.5 

 inches; inside rods, from pin to pin, 24.25 inches; from 

 the pinning of the lower end of the outside rods to the 

 centre of the ball, 5 inches; making in all 90.25 inches 

 of steel, to be compensated by the brass. Outside brass 

 rods, from pin to pin, 26.87 inches ; inside ditto, 22.25 

 inches, being in all 49.12 inches of brass. The length 

 which the brass ought to have may thus be found by 

 the inverse rule of proportion. As 90.25 inches is to 74, 

 the number for the expansion of steel according to Ber. 

 thond, so is 121, that of the expansion for brass, to the 

 length of brass required; that is, 90.25 X 74 -=-121 =49.4 

 inches, the" length required. Although the deficiency 

 of brass here is very little, yet to remedy the compen- 

 sation, a greater number of inches, both of steel and 

 brass, must be taken, before this pendulum can be made 

 complete. A description of such a one shall now be 

 given. 



The length of the outside steel rods, from pin to pin 

 in the uppermost and lowermost traverses or brass cross 

 pieces all, cd, is 86 inches; the next or innermost steel 

 rods, from their pinning in the second uppermost tra- 

 verse m, to that in the second lowermost n, is 35 inches ; 

 the steel centre rod, from the pinning of it, in the third 

 lowermost traverse o, to the upper end of the pendulum 

 spring, is 37 inches- and of an inch, or nearly 37.6^8 

 inches ; the centre C of the ball below the pinning of the 

 outside steel rods and index rod, is 3.94 inches; the outer- 

 ino-it brass rods, from their pinning, in the uppermost 

 traverse to that in the second lowermost, is the smallest 

 quantity possible less than 35.5 inches ; the innermost 

 brass rods from their pinning in the second uppermost 

 traverse to the third lowest one, is 154.5 inches, The 

 whole length of the steel is then 1 I2.5fi<s inches, and 

 lhat of the brass 70 inches. The diameter of the rods 



' 



is a quarter of an inch each. The distance from the 

 centres of the two outside steel rods, is 2.5 inches. The 

 rods are placed equidistant from one another, only there 

 is a little more space left between the two brass rods Gridiron 

 nearest the centre and the steel centre rod, in order to pendulum. 

 give room for the fork to come in and clip the centre PLATE 

 rod. The two outside steel rods are prolonged below CCCVII; 

 their pinning in the lowest traverse, as seen below c d, 

 about five or six inches within the ball, in order to keep 

 it properly flat in the plane in which the pendulum 

 should swing. In the centre of the lowest traverse c d 

 is pinned a steel rod ef, somewhat more than a quarter 

 of an inch in diameter, and about nine inches long. This 

 rod goes through the centre of the ball, the index point 

 f being on the lower end of it, and it is tappe I for an 

 inch in length at that part which lies near to the centre 

 of the ball. A cross piece of brass is fixed to the inside 

 of the ball before casting it, the lowest side of which is 

 in a line across the centre horizontally. The ends of the 

 two outer steel rods, and the centre or index rod, come 

 easily through this cross piece of brass. A hollow tube 

 comes up within the ball, as far as the underside of 

 the i iside cross piece, on the end of which, where the 

 cross piece and ball rest, is fixed within it a tapped 

 nut, which screws on the tapped part of the index rod. 

 On the lower end of the tube is soldered a sort of flat 

 conical head or nut It, nerrelled on the edge outside, and 

 whose diameter may be an inch or an inch and a quar- 

 ter. On the upper surface of this nut are traced two 

 circles, in order to put divisions between them, and fi- 

 gures so as to correspond with the turns of the screw 

 in an inch. A small steel index z is screwed on to the 

 lower part of the ball, to point at these divisions. The 

 lower end of the tube is a very little below the edge of 

 the ball, that it may rest freely on the upper end. The 

 total length, from the upper end g of the pendulum 

 spring to the index pointy) is 47.75 inches, a length of 

 radius which will require the length of a degree on the 

 index plate to be .833142 of an inch. The distance 

 from the upper end of the pendulum spring to the cen- 

 tre of the ball, is 42.5 inches very nearly, so that the 

 centre of the ball is about 3.3 inches below the centre of 

 oscillation. The lowermost traverse cd may be about 

 half an inch thick, and its length and breadth such, as 

 to give it sufficient strength to receive the outside steel 

 rods, and the centre or index rod : The uppermost tra- 

 verse a bis nearly of the same dimensions. The second 

 lowermost traverse n, and the second uppermost m, are 

 nearly of the same size, and almost equal to that of 

 the uppermost and lowermost ; only they are a little 

 shorter, having in their ends a sort of half hole, through 

 which, in the second lowermost traverse, the outer 

 steel rods pass easily ; and through these, in the second 

 uppermost traverse, the outside brass rods freely pass. 

 This traverse is, of course, a little shorter than the se 

 cond lowermost. In the second lowermost traverse is 

 pinned the ends of the outermost brass rods and in 

 the second uppermost traverse are pinned those of the 

 innermost steel and brass rods. In the third lowest tra- 

 verse is pinned the innermost brass rods and the centre 

 steel rod : The dimensions of this are nearly the same as 

 the second uppermost audio vvermost, only shorter, having 

 a half hole at the ends, through which the inner steel 

 rods pass freely. There is a hole in the middle of each 

 of the two uppermost traverses, through which the cen- 

 tre rod can pass freely. Towards the lower ends of the Method of 

 centre steel rod, and those of the innermost brass rods, 5juting 

 are two sets of holes, by which the third or lowermost 

 traverse can be moved or shifted up either three or six 



