RESULTS OF BRITISH TRANSIT EXPEDITIONS. 75 



Nevertheless, it must not be forgotten that on a former 

 occasion when equal satisfaction was expressed with the 

 result of a rather less costly but still a laborious and difficult 

 experiment, the scientific world did not accept (and has 

 since definitely rejected) the conclusion thus confidently 

 advanced. I refer to the famous Harton Colliery experi- 

 ment for determining the mass of the earth. The case is so 

 closely analogous to that we are dealing with, that it will be 

 instructive briefly to describe its leading features. Maskelyne, 

 formerly the chief Government astronomer of this country, 

 from observations of the effect of the mass of Mount Sche- 

 hallien in deflecting a plumb-line, had inferred that the 

 density of the earth is five times that of water. Bouguer from 

 observations in Chimborazo, and Colonel James from obser- 

 vations on Arthur's Seat, had deduced very similar results. 

 From pendulum observations on high mountains, Carlini and 

 Plana made the earth's density very nearly the same. Caven- 

 dish, Reich, and our own Francis Baily, weighed the earth 

 against two great globes of lead, by a method commonly known 

 as the Cavendish experiment, but really invented by Michell. 

 These experiments agreed closely together, making the 

 earth's density about 5 \ times that of water, or giving to the 

 earth a mass equivalent to that which would be contained in 

 6,000 millions of millions of millions of tons. Now, from 

 the Harton Colliery experiments, in 1854, in which the 

 earth's weight was estimated by comparing the vibrations of 

 a pendulum at the mouth of the mine with those of a simi- 

 lar pendulum at a. depth of about 1,260 feet, it appeared 

 that the earth's density is rather more than 6^ times that of 

 water, corresponding to an increase in our estimate of the 

 earth's mass by nearly 1,100 millions of millions of millions 

 of tons, or by more than a sixth of the entire mass resulting 

 from the most trustworthy former measurements. Sir G. Airy 

 considered that f this result will compete on at least equal 

 terms with those obtained by other methods ;' but nearly a 

 quarter of a century has passed during which no competent 

 astronomer has adopted this opin on, or even suggested 



