TRANSACTIONS OP THE SECTIONS. 73 



the boss. To set this part of tho instrument, it is mei-ely required to bring one 

 liand in contact witli eitlier side of the rod ; the distance to which they arc parted 

 denoting the amount of variation in tlie ^vind. 



This wind-gauge may prove useful in rifle practice and on numerous other occa- 

 sions when it is importixnt to bo acquainted with the actual pressure of the wind. 



Meteoroloc/ical Observations at Hur/gate, Yorlisldre. Bij the Rev. T. Eankin, 



This was a continuation of meteorological tables and notes of weather and all 

 remarkable meteorological occurrences during the year 18G0-G1, which the author 

 has annually presented to the Association for upwards of twenty years. 



On a BatJiometer, or Instrument to indimte the Depth of the Sea on Board 

 Ship luithout submerging a Line. Bij C. W. Siemens. 



Those who are acquainted with the difficulties and expense attending tho taking 

 of deep-sea soundings bv means of a weighted line, will readily perceive that an 

 instrument capable of inc\icating depths upon a graduated scale without submerging 

 any apparatus would be of great advantage as a means of extending our knowledge 

 of ocean geography. In layiug submarine telegraph cables through deep seas such 

 an instrument would certainly bo invaluable. 



It occurred to Mr. Siemens that the total attractive force of the earth must be 

 sensibly influenced by the interposition of a comparatively light substance, such as 

 sea-water, bet'ween the vessel and the solid portion of the earth below. This he 

 demonstrated geometrically as follows : — 



Assuming the earth to be a perfect sphere of unifomi density, two lines are drawn 

 from a point on the surface, so as to intersect the circumference at the semicircles. 

 A line is then dra'wai through the two points of intei-section, which passes through 

 the earth's centre, and a second line parallel to it, touching the circle at its lowest 

 point. It was next demonstrated that in dividing the solid cone represented by 

 these lines into a number of slices of equal thickness, in a direction perpendicular 

 to its axis, each slice woiild exercise the same amomit of attractive force upon a 

 body at the apex of the cone, the reason being that the mass of each slice increases 

 in the proportion of tho square of its distance from the apex, and the attractive force 

 diminishes in the same ratio. It was thus demonstrated that the true centre of 

 gravity of the earth, in reference to an attracted body on its surface, does not reside 

 in its geometrical centre, but in a variable point between the centre and the attracted 

 body. In dividing the sphere itself into slices of equal thiclmess, a mathematical 

 expression was obtained representing the attractive force of any of these slices ; and 

 in integi-ating this expression for a series of slices commencing from the point of 

 attraction, a fonnula was amved at, showing that for moderate depths the attrac- 

 tion of the earth may be represented by a veiy obtuse cone with two-thirds of the 

 earth's radius for its height If sea-water were of no weight, the total attraction of 

 the eai-tli would be diminished upon its surface in the proportion of tho depth to 

 two-thirds of the earth's radius ; but considering that sea-water has about one-third 

 the weight (bulk for bulk) of the generality of rock, the actual diminution of gi-a- 

 vitation was sho-\ra to take place in the proportion of the depth to the radius of the 

 earth. 



Accordingly 1000 fathoms of depth woidd produce a diminution by -jaV^*^ P^rt 

 of the total gravitation — a difference so small that it appears at tirst sight impossible 

 to construct an instrument capable of indicating it with sufficient accuracy. 



The second part ot the paper described the instrument designed for this pui-pose, 

 which consists of a tube containing mercury, diluted spirits of wine, and coloured 

 iuniper oil. The mercuiy column, about 30 inches high, ascends in a tube from the 

 bottom of a large bulb containing imprisoned air, and terminates in the middle of a 

 second bulb. The remainder of the second bulb is filled with the diluted spirits, 

 which reach upward into a narrow tube provided with a scale. Upon this rests a 

 column of the coloured oil, which terminates in a third bulb, — the remaining space 

 being vacuous, or nearly so. This gauge is enclosed in a glass tube fllled with 

 distilled water, which in its turn is surrounded with ice contained in an outer 

 casing. The latter is suspended by a imiversal joint. The air in the lower bulb 



