TRACT 26. MEAN DENSITY OF THE EARTH. I 1

Of the measurement of the base afiy in-Hannoch, to the North-west of the hill of Shichallin.

1. One part of this base was measured twice over, in dif-ferent ways. The part a/3 was carefully measured on the 8thof October 1774 with a chain, and found to be 63 chains and401- links, or 63-405 chains in length.

Now on the 24th of the same month the chain itself wasmeasured, by means of the five-foot brass standard, when thethermometer was at 38°r, and the length found to be 65-94542feet. Hence then 65-94542 x 63405 = 418 1 -269, is the lengthof all the chains ; to which adding 1-764, the breadth of the63 iron pins, the sum is 4183:033 for the length of a/3 un-corrected.

But 62—38 a = 23j ; therefore - 23-'- X X 4183= —

1'05C, is the reduction on account of the state of the thermo-meter, which being applied with its proper sign, there results4181-977 ; and from this last number deducting again theTTB-crot-h part or -232, on account of the wearing of the brassstandard, there then remains 418P745 feet, for the length ofthe part a/3 of the base in Ilannoch, as measured by the chain.

But as the chain was measured not at the same time withthe base, but between two and three weeks later, when theair was probably cooler, the reduction above made for thestate of the thermometer is perhaps something too great, andwe may safely conclude a/3 to be equal 4i82 feet as measuredby the chain.

2. The whole base a/3y was next, on the 10th, 11th, and12th of October, very carefully measured by the twenty-footmeasuring rods. The rods at that time measured thus,a = 20 feet f T306 inch. = 20-108^^

B = 20 feet + T354 inch. = 20-112-5- j ec ’the thermometer being then at 40°. The number of rods andthe additional parts were as follow-.