36 PHILOSOPHICAL TRANSACTIONS. [anNO 1744. 



of which, as they would imbibe more moisture, would make the machine less 

 accurate ; and the other brachium he charged with a sponge, suspended likewise 

 by a thread, of such a weight, when reduced to absolute dryness, as made an 

 equilibrium ; and then screwing on the index to the first degree of the 180, and 

 exposing the machine, thus ordered, to the open air, in one night's time the 

 index had got to the 70th degree ; which, as the sponge had been absolutely dry, 

 must have been the true state of the air, as to moisture, at that time. 



He found this machine extremely sensible and accurate ; it would alter 10 de- 

 grees in a night, and as many in a day. 



The near correspondence between the degrees on the graduated plate, and 

 the weight of the moisture necessary to be imbibed or exhaled, to make either 

 brachium of the balance preponderate every such degree, gives it the preference 

 to any other. For a more perfect idea of this machine, see fig. 12, pi. 1, where 

 it is viewed on the inside, the back being slid up. At fig. 13 is represented the 

 plate with its graduations and index, as it should appear on the front of the case. 



Of the Anemoscope, fig. 14, pi. 1. The Anemoscope is a machine 4-j- feet 

 high, consisting of a broad and weighty pedestal, a pillar fastened into it, and an 

 iron axis, of about half an inch diameter, fastened into the pillar. On this axis 

 turns a wooden tube, at the top of which is placed a vane, of the same materials, 

 21 inches long, consisting of a quadrant, graduated and bound with an iron rim, 

 notched to each degree ; and a countei-poise of wood, on the other. Through 

 the centre of the quadrant runs an iron pin, on which are fastened 1 small round 

 pieces of wood, which serve as moveable radii to describe the degrees on the 

 quadrant, and as handles to a velum or sail, whose plane is one foot square, 

 made of canvas stretched on 4 battens, and painted. On the upper batten, next 

 to the bound rim of the quadrant, is a small spring, which catches at every 

 notch corresponding to each degree, as the wind shall, by pressing against the 

 sail, raise it up ; and prevents the falling back of the sail, on the lessening of 

 the force of the wind. At the bottom of the wooden tube is an iron index, 

 which moves round a circular piece of wood fastened to the top of the pillar on 

 the pedestal, on which are described the 32 points of the compass. By which 

 this instrument shows both the direction and strength of the wind. 



The Ombrometer, or Rain-gauge, fig. l6. — This machine consists of a tin 

 funnel, its surface an inch square, a flat board, and a glass tube let into the 

 middle of it in a groove, and an index. His board was about 3 feet long, to 

 answer the height of the rails that go round the top of his house, to one of which 

 it is hung, clear of any obstacle to prevent the free fall of the rain, with 4 small 

 staples that slide over as many tenter-hooks. The bore of the tube is about half 

 an inch ; which, at a medium, is the best size, a larger bore obliging you to make 

 your graduation the more contracted, and consequently the less plain and accu- 



