The People of Gibraltar
1771 - Healy's Mortar - Great was the Explosion

Lieutenant Colonel Green and General Lord Cornwallis
Captain J. Phipps and Lieutenant Colonel Ramsay
Captain Benttinck and Colonel Boyd

Healy's Mortar - an odd bit of weaponry hewn out of solid rock - is situated just north of the Ape's Den near Queen's Gate on the Upper Rock. 

Tourist plaque near Healey's Mortar

Cutting a mortar out of solid rock was not an original event in Gibraltar. There was one already in existence below a fountain in the area now known as the Alameda Gardens. According to local historian George Palao the mortar had probably been constructed by either the Moors or the Spaniards just after the arrival of gunpowder in Europe and anywhere from the late 14th to the 15th century.  But Healy's was much bigger - and had the advantage of being better known. It was British.

It is hard to tell who Healy was - he is usually referred to in the literature as Mr. Healy - but he may have been a civilian working for Gibraltar's chief engineer Lieutenant Colonel Green. If so Mr. Healy didn't last long. Less than a year later Green became fed up with the difficulty of imposing military discipline on his civilian employees, created the Soldier Artificer Company - the future Royal Engineers - and got rid of the lot of them. ( see LINK

Whatever the case Healy built his mortar in 1771. For anybody interested in how the thing was constructed, the finer points are described below by a certain Lieutenant Colonel Ramsay of the 30th Regiment.
The mortar is cut out of a rock, which Mr. Healy pitched upon, about 200 yards higher than the level of the sea, and 400 yards horizontal distance from the line wall. He began by forming a plain surface of 45 degrees elevation, then bored a centre hole, or axis, four feet deep, and perpendicular to the said surface, from that centre, described a circle three feet diameter;  the rock being so hard that he could not excavate it by chipping, was therefore obliged to bore holes all round the circle, each inclining to the centre, so that all those holes run into one another, and into the centre hole near its bottom. .

Healy's Mortar ( Adapted from Scott B. Anderson )
Then cutting away the partitions betwixt the holes, a core of a conical figure become loose, and was extracted, which core Captain Benttinck brought home in the Centaur; Healy's next care was to chip off that conoid, so as to form it into a true parabola, and then to polish it. 
As the nature of a parabola is such, that either light or sound sent from its focus proceeds in parallel lines; so he concluded that the impetus given to any charge by the explosion of gunpowder would be also parallel; doubtless it would be so, but as his mortar was loaded brimful of stones, those which lay close to the tompion being first impelled, must of course strike those before them variously, by which means, like billiard balls, the foremost must pursue the direction in which they were struck, whence their spreading to the right and left, a good distance, is obvious, and rather an advantage than otherwise, where a great body of men are supposed to be marching, or a great number of boats are attempting to land troops. 
This mortar has no other chamber but the bottom of the parabola, nor touch-hole but the hollow reed and copper tube which convey the fire from the muzzle to the focus, down through the very centre of the stones and tompion; by this contrivance the copper tube terminating in the focus and centre of the powder, the whole being instantly kindled, acts more forcibly than when lighted on one side, as in cannon, mortars, etc.
Healy's Mortar  ( 1975 - George Palao - Guns and Towers )

The mortar was tested on the 14th of May 1771 and as authorised by General Lord Cornwallis who was Governor at the time. The following is a summary of the lengthy proceedings as recorded three days later by Captain Phipps - another engineer stationed in Gibraltar - in a letter to Lieutenant Colonel Ramsay.
Agreeably to promise I sit down to write a few lines relating to Healy's mortar; the 14th instant was at length fixed for trying that new invented pierrier; eight in the morning was the time ordered by the governor; South Port Gate was shut, and the three guards between that and the South Barracks retired out of harm's way; the general had referred the charge of the mortar, to Colonel Philips;  
Healy stood out for 50 pounds of powder, but the colonel insisted that the first experiment should be with 27 pounds; proper paving stones were collected, and a detachment of artillery attended to assist in loading; after the powder was placed in the piece, a tompion, or bottom of wood, was carefully put to cover the charge, a copper tube conveyed the quick-match from the tompion to the centre. 
The stones were carefully put into the mortar, 147o in number, the least a pound weight, and few exceeded 1¼  lb; a hollow cane well directed conveyed the quick-match through the stones to the copper tube, and upon the extremity was fastened a port-fire, to burn five minutes before the fire should reach the quick-match ; when the port-fire was first  lighted, and every one retired to a great distance, with various conjectures about the success of the machine, five minutes passed, ten minutes passed, no explosion! 
Poor Healy very impatient; at length people approached nearer and nearer to inquire into the mortar's silence, when upon examination it was found that the port-fire had but half burned; by some accident it was choaked-in the-making: another was immediately applied, which had the desired effect.
Great was the explosion; near a quarter part of the stones went into the sea, above 1oo yards; to the right they extended as far as Ragged Staff and to the left as far as the Watering Pier, but no damage was done; the cavity of the piece was searched, but no fracture could be seen, and what was surprising  the rock above that appeared so bad did not give way;  it was fired a second time with the same charge, the success much the same as the first. 
 The third time it was loaded with 13 ½ lb. (viz. half the former charge) and 1220 stones, when I suppose about 200 passed over the line wall; with these three discharges the eight-gun battery was covered over' with stones; she spit her venom most there; after three experiments Healy received the congratulations of Colonel Boyd and all the officers upon the road; the general took his observations from the terras walk; 
the mortar had not received the least damage; its complexion is a little changed by the powder. Colonel Boyd who goes home in the Lizard,  which sails the first Levanter, hopes to see you in London, to give you a particular account of the success of your friend Healy, who wished; several times you were present . . . .  
It may not be improper to remind you that the figure of the mortar is a parabolic conoid, length of the axis. four feet, and the diameter of the bore at the muzzle thirty six inches, and the solid content fourteen cubical feet; I am, Sir, with great esteem, Your most obedient, and very humble servant, 
 J. Phipps
The only other 18th or 19th century reference to Healy's Mortar I have been able to find is in - of all places - The Tradesman or Commercial Magazine of 1811 in an article entitled - Interesting Account of the Present State of Gibraltar. In it the anonymous author states;
Queen's Gate  . . . and near this is a very large mortar curiously formed in the rock, commanding the dock-yard, and fired by a train.'
All of which suggests that the tourist plaque shown at the site is rather unfair to Mr. Healy. I have no idea if the mortar was ever again fired in anger - it was definitely not used during the Great Siege a few years later - but never seems to have been fully tested with Healy's own estimate of 50 lbs of powder.

In any case whatever its apparent shortcomings his immediate superiors - including the Governor and Colonel Boyd - seem to have been quite pleased with its results.